I agree, the MT15 really has a pristine clean channel that takes effects like a champ. I also strive for a crisp and detailed sound, be it clean or lead channel, and because of that I also fiddled around with different preamp and power tubes and with speakers as well, that is why I paired the Jensen with the V30, it is a bit more tame(breaks up a bit later) than having two V30s in my 2x12 cab ... although for a purely hc/metal/heavy sound the V30s are an awesome combination with the MT15. For jazz/blues/country you could also try a 5771 or 12at7/12au7 in preamp positions 1,2 or 6, they are a bit smoother ... if you should need less power.
How is your FX loop, do you have any issues with the hum/noise when using it? I had some issues with that before I changed some tubes, have to test again to see if anything changed, because I noticed that a bad preamp tube can make quite some noise.
I have seen other threads and comments about MT15 effects loop noise. I use the effects loop every day for years and never noticed any noise. In response to your question, I unplugged my effects loop as a quick test, and found that there is a tiny hum with the effects loop connected, but honestly I don't know if the hum is coming from the loop in the amp or the effects in the loop. Regardless, even while playing clean, the hum is not bad.