i gotta wonder what your post knobs are at? if you bought this amp to be a bedroom warrior, you made a HUGE mistake. most of the tonal "issues" you are describing are always going to be an issue at bedroom volumes with this amp. throwing money at the problem may or may not address your specific tonal needs.
my advice? turn the volume up and adjust your tone and pre gain knobs. add presence and resonance to taste. see if you can get that with lower volumes.
generally, the 6/6/6 tone knob settings is a good place to start, providing your lead and rhy post gains are at least at 2...and above that. which, again, isn't bedroom/apartment friendly....its really fucking loud, but the tone starts getting sweeter as you up the volume. there's a few mod tricks you can do to address the out of control volume, but, honestly, you should be able to get there tonally with bearable volume levels.
you can always add the 10band in the loop, this usually cures most problems i hear about people having with the limited tonal capabilities of the 5150/6505v1 series. you run a boost in front, which tells me you're willing to try a 70$ eq pedal in the loop to, if need be....this may just be your start and end point.
after around 3ish on the post gains, the presence wakes up a bit earlier on the dial before 7...but yes, generally, that's the starting point at bedroom volumes where you start hearing a difference. again, YMMV a tad from amp to amp, but generally, that's where the wake up is at. the 10k presence knob in there is just about as low a knob you can get (i've tried 50k and 25k with the same "around 7" results), and they put it in there for a reason. you can put a 3-4k resistor across the presence knob from wiper to ground on the capacitor side to even out some of the presence knob sweep, but keep in mind, whatever value you choose will be the starting point now for the knob....still not a bad mod, but not the be all end all cure all...but it's better than stock.
i run my amp with x patterned celestion t75 and g12h100 (similiar to the k100) and i have no problems with fizz. your speaker/cab choice could be the issue. i dont know, as i currently dont have v30's to a/b against w my amp, and certainly, i dont have your ears/nor you mine. then again, as was said too....the fizz dissapears with these amps in a band setting, and the roar comes to life, and a lot of people do use these amps with mesa cabs/v30s.
sovtek wxt power tubes sound good in these amps.
chinese 12ax7b v1/2/4/5 and a sovtek 12ax7LPS in the PI slot (v4) should get you tonally and gainially, about the maximum you can have, and be able to make the most adjustments with your knobs in a 5150, without starting to split too many preamp tube hairs, etc.
one or two 3mica 5751 tubes may also serve to tame some high end and lower the gain a bit, which isn't a bad thing in the 5150....there's so much pre gain on tap between 2-7 on the lead side and across the whole knob on the pre side, you shouldn't really be maxing this out to begin with. i went a different mod route instead of 5751 tubes as they're a tad pricey compared to a few other options.
