I've posted in a few threads about the UA and mentioned that I was having some issues with a crossover type distortion sound as soon as I started to push my amp a bit. I have a 65 bassman that wants a 4ohm load so obviously there is some concern with pushing it through a nonreactive 30ohm load. I replaced all of my tubes as it just needed to be done but the problem persisted. So I contacted Mark through email, explained the problem that I was having and sent him a rough sound clip. He thought that it might be caused by what he called the plexi circuit in the UA. I thought that it was coming from my amp, but he seemed pretty sure. My UA came without the "plexi-switch" option but he gave me the option of installing it myself if I didn't want to send it back to him. I can pretend to use a soldering iron so he sent me some instructions on how to install it to bypass the plexi business. After a few minutes and a couple "Hey Honey, will you hold this for a second"'s I had the switch installed. Flipped it on, turned the plexi circuit off with my new switch and the fuzz is gone! It worked. I finally have the deep, rich clean that I was shooting for with out the sizzle. I love the overdrive of my amp so much I just use a compressor to push the front end of the amp and get a killer tone. No TS9 required althought they play nicely too. Turn the amp up to 6 and I have a great smooth distortion with that beautiful fat midrange that works magic with my Fralin Steel Pole 42. I'm in love with my amp all over again and I'm sure my neighbors will be happier too since the sound is virtually the same at any volume minus the speaker interaction of a pushed amp. Everyone talks about how well the UA works with the amp dimed, but it works beautifully with a chimey fender tone as well. Just make sure you get the Plexi-switch. You can get a nice clean tone with the amp turned down on it's own, but the sound can be thin and you don't get that midrange push that adds a bit of fatness to the notes to keep them from sounding too brittle and that natural tube compression. So people start buying an army of tubescreamers and are disappointed that the overdrive doesn't sound natural. Now I can overdrive the amp it self and it just roars. Now all of that said I will admit I'm not entirely comfortable running the bassman at such a high load, my amp doesn't have a lot of ventilation around it and it can get very hot if I play for long enough. So i'm looking into getting one of the weber Zmatchers to take some of the load off so to speak. I'll report back on how that works when I get it. Will probably be a couple of weeks.