Bear with me, this is way long, as I want to include all pertinent info! The main problem is a hum through my SVT-15E when I biamp, but not otherwise. I love the amp, it's loud as anything I've heard, and I can 'live' if this problem isn't fixed. But it's one of those things that I'd love to have answered... details follow! I'm running a Modulus 5-string through an Ampeg SVT-4PRO with two cabs, one is the 1x15, one is the 410HLF. I had been running the 410HLF mono-bridge and it works great and is loud, but wanted to add a little depth to the low B string, so I experimented at Guitar Center and tried adding the 1x15 cabinet. I liked the tone with it running stereo, but LOVED it running biamped. The manual shows a diagram running a shielded cable from "biamp low out" to "power amp A in (low)" which I did, trying several types of cables, all of which worked, but there's that semi-quiet, but definitely noticable, hum, only when biamping. When I run the two cabs in regular stereo mode, the hum goes away, and there's the usual super-quiet speaker hiss. I have tried various cables, and another interesting note is that it will do this regardless of whether the amp is muted, and even when the bass is not plugged in. It's not a big problem, because when I'm playing it's not noticable, but when it's sitting there it's kind of bothersome as one of my venues is a church, so when stuff is quiet, the amp isn't. I could just run stereo, but I lose the crossover point and balance feature, which when I run biamp gives it such a great, creamy tone, and still allows good slap/pop action with the pop not being overplayed by the 15. I have tried two different power conditioners (one cheapo, one by Monster which is supposed to be really good); I have tried cheap and expensive cables to bridge, and there's no difference whatsoever. One little interesting piece of info is that it buzzes immediately upon connecting to the "power amp A low" input on the back. Also, I tried a couple other duplicate heads at GC and they do the same, so it's not just my head. My other question is: is the Guitar Center guy right in saying that it's okay to biamp with the 15 cab on Power Amp A at 8 ohms and the 410HLF on B at 4 ohms? I know it wouldn't be so hot to mono-bridge them together (2.67, right?) so I'm not doing that, but they said it was 'no problem' to biamp or run these two cabs stereo. I do have time where I could return the 15 and get my cash back if they were wrong about the impedance - so far nothing has overheated and blown up, though. I would just return the 15 and mono-bridge the 410 (which is great) but it sounds SOOO good biamped except for the hum. I mean, people walking up in the store to see what combinations we're running and stuff. Any advice or experience with this?? Ampeg Insider any suggestions? Thanks in advance everyone! I'll be happy to supply any more info if needed!