Well, after beating on my YBA for the better part of a year or so now, I finally brought the head in for new power tubes. I went with a quartet of JJ KT88s to replace the standars issue Sovtek 6550s. Keep in mind, I've already swapped my preamp tubes, a 12AY7 in V1, a 12AT7 in V2, and the stock 12AU7 in the splitter. Anyway, I tested the amp with it's new power section last night.... The down side: I actually noticed a *decrease* in clean headroom with the JJs. I can still get the amp quite loud, but breakup seems to come on sooner, regardless of whether the gain is up - master down, master up - gain down, or master and gain set similar. When it comes to bass amps, I love tube WARMTH, but not overdrive per se... I was a little disappointed in this regard... The up side(s): These JJs compress *beautifully.* Where the Sovies overly accentuated particular notes and frequencies while seemingly ignoring others, the JJs deliver consistent volume and response across strings and up and down the fretboard, while still offering remarkable touch sensitivity. Another plus is the overall tone: While the tone thus far has a little less presence than the Sovies, it is indeed warmer, rounder and definitely more SVT-like. The sound now has the impression of being easier on the ears, while more noticeably *felt* in the chest and the pant legs, as it were. Lastly, the overdriven sound of the JJs hands-down beats the Sovteks. Now - for MY tastes - the less distortion, the better, but if it's gonna happen, it might as well sound good. The YBA200 sounds more like a vintage YBA-3 when pushed into overdrive with the JJs. Nice sound, if that's your bag. Another note to add... My amp tech is Danny Russel of Blitz Amps in St. Claire Shores, MI. When Danny has some insight to share, I listen - those of you out there who know him or know who he is know why. He informed me that the "warble" I reported when pushing the amp with the original tubes was actually the beginning of clipping. He said that the design of the amp circuit is such that, unfortunately, it doesn't allow for the smoothest of transitions into overdrive, and it simply sounded particularly objectionable with the Sovteks. I have noticed that that transition from clean into clipping is still somewhat harsh, but it's much more tolerable with the JJs. To wrap it up: A little disappointed with the slight reduction of clean headroom, though I'm more satisfied with the overall tone and behavior of the amp. If your YBA is newer and healthy, there's no reason to run right out and get KT88s installed. I still have plenty of knob-turning to do before drawing complete conclusions, and perhaps I'll try out yet another preamp tube combo to find the optimal front-end compliment for the JJs.