One man's trash is another man's treasure!
Ok, that's taking it to extremes. But, what you dislike about your current space is actually what I had been hoping for when we were house hunting, and in a way makes for a better recording space. The fact that you have a generic basement room with some size makes it easier to do the things you've done like adding sound treatments, fancy ceiling, etc., without feeling like you're taking away from a room that already had its own thing going on.
What my room has going for it, is that it's a fantastic work space. I have those nice built in cabinets to store gear (hide new pedals and mics from my wife) in, which keeps everything at arms reach, and the nice big desk where I can do digital work as well use as a work bench when a guitar needs work. Sonically, however, it's not that great. It's too small to get the amps away from the walls and corners, which is an especially big deal with my open back combo. I can get them to an acceptable place in the room, yes, but it's in the walkway to the mother-in-law suite. Fortunately, I scored a very likeable set of in-laws, so I don't mind moving amps for her! It's also such a small and boxy room that things like vocals or acoustic guitar catch a lot of "small room" reverb and reflections, which I don't care for. I have a buddy who works for an acoustic engineering and AV design company, and he's taken it upon himself to figure out the funky multi-directional ceiling treatment we can put up, and bass traps, acoustic deadening on the walls (don't expect me to know what the technical terms are for this stuff), but in my mind all I see is my room getting smaller and smaller as we try to correct the acoustics, as well as just ruining that vibe of being a fine looking room and great workstation as-is. That's why I've decided that when the time is right I'm going with the UA OX. I could spend a lot of time and money wrecking the vibe of my workspace in search of one decent sounding room, OR grab that piece of gear and get a much wider variety of options, likely of much better quality than anything I could do with this physical space at ten times the price. When I consider the wide array of speakers and cabs, mics, rooms, and other features of the OX, and my conclusion that the quality of sound is there - perhaps not as good as a very nice studio, but like I said, better than I could achieve myself the old school way at many times the price - it's just the most logical choice.
Anyways, my point is, there's something to be said for the big, generic room that you can have your way with. I had wanted such a room for recording, and so I could set up a band rehearsal space for 5-ish pieces, with an electronic drum kit and powered monitors for all, and I didn't get that. What I did get, I like just as much, just in a different way.
My former space was designed more like yours, with built-ins, etc, though because I had it built, I got to decide what went where. It looked fantastic, sounded nice. Oddly, though, my current space looks a lot worse, but sounds better, and who the heck even knows why? I suppose an acoustician could figure it out, but I sure can’t! It was larger than my current space, too.
Narrow spaces can be a bit tough to treat acoustically, and it’s more of a problem for mixing and evaluating tracks than for tracking, simply due to the reflections at all frequencies that the monitors reproduce. If the ceiling is below 8 feet (don’t know if yours is) as your friend says, that can probably exacerbate the issues.
One of the first things I’d do in your room is elevate and decouple your monitors from the desk surface. Speakers sitting on a desk will cause it to vibrate sympathetically, and you can hear it. If they’re decoupled, you’ll have better imaging and truer bass response. The most effective decouplers I’ve found are the Isoacoustics line. They make an adjustable-height stand that is a very effective decoupler, and they also make some “pucks” that won’t elevate a speaker much but will decouple it very well (I use these, because my speakers were already at the right height on stands behind the desk). When their line was introduced, I had a few phone conversations with their owner/designer, who also was the acoustical designer for Canadian Broadcasting’s studios. He knows his stuff, and I wound up throwing out the foam-based stuff I previously used. The difference was amazing, actually. When I recommended these to my partner and to my son, they were astounded enough to call me and sing their praises.
Having speakers elevated, with your ears just a little below tweeter level, will also position them so you hear less room and more speaker, which is the main point of near-field monitoring. I will say that my son, who’s mixed several major-label records in his own very basic studio room, uses the Isoacoustics stands as well, but likes his tweeters a little below ear-level; why, I don’t know, but everyone needs to work their own way. I’d still start with them at ear-level and make adjustments from there if you find it too bright. If you want to figure out where you hear the most detail, just elevate them on some books to figure out what height to use.
One of the great things about bass trapping is that, unlike reflection-solving, it doesn’t matter quite as much where you put the bass traps, because bass is more omnidirectional. And the bass doesn’t know or care if the traps are on the ceiling, the floor, the walls, etc. Yes, they work best straddling corners. Membrane traps work better than foam or fiberglass-only. Any bass trapping is better than none.
The first place I’d put them in your long, narrow space is in the corners of the wall behind the speakers. You might think that would look terrible in your room, but you can get off-white 2’ x 2’, 4” thick traps and simply rest them on your desk in the corners when mixing or tracking, and take them off the desk when you’re done, keeping the room looking nice when you don’t need to use them. If need be you could add one in the center of the front wall on the desk, too. 2x2 traps thick enough for free-standing are available from companies like RealTraps, and they’re not expensive. They are very light in weight!
As a HUGE plus, you can also put them in front of an amp as gobos, to reduce volume in the house, and to prevent a microphone from picking up room reflections. I have a pair of these I use for just that purpose sometimes, and then I use them as bass traps in the corners behind the amps at other times.
In terms of side wall reflections, RealTraps (and probably others) make stands for their acoustical treatment so that it can be positioned temporarily.
To make the most significant improvement in your mixing, getting the imaging and midrange/hf balance right, you must absorb the first reflections off your monitors. Permanently installed acoustical treatment would indeed mess up the look of your wall space when you’re not mixing. The beauty of stand-mounted treatment is that when you’re done mixing, as with the small corner traps I’ve suggested for your desk, simply put them away somewhere.
You can get a look at some of these concepts and some good info about getting good real-world acoustics on the Real Traps site. I like their stuff because it’s very easy to use, the nice folks will help you choose what you need, it’s inexpensive, and it’s very, very effective. Also, with things like gobo traps, and stands, they “get” what it’s like to have to deal with a home studio’s issues, so permanent mounting isn’t necessary to take advantage of their traps.
However, I’ll bet other companies have similar stuff, so it’s worth checking around.