Krell Theater Amp Standard problem


I have hum noise and low volume gain problems with a Krell TAS (bought through Audigon) in a home theater setup and need some pointers before send it back to Krell for service. The set up:

Krell TAS driven by Krell HTS 5.1
PS Audio XLR interconnects
Vienna Acoustics speakers

I bought the Radio Shack SPL meter to do the sound level adjustment. There was one channel that measured about -28/29 dB compared to the other four channels. I could set the trims to -15dB (on 4 good channels) and +14dB (on the problem channel) to get a balanced and overall ~60dB sound level (the manual recommends 75dB level) but the sound didn't come out quite right (I used Yamaha RX2700 as preamp before and it sounds better). I have reset the HTS to factory settings but that didn't solve the problem.

Another problem was a hum noise that occurred in one good channel. It measured 56dB so it was quite loud and irritating. The funny part was that it was in another channel before - I haven't figured out the reason for it to switch.

I have tried to change the preamp inputs around but the problems persisted - so this eliminated interconnect and preamp problems.

I now have my theater run by the Yamaha RX2700 drives the L/R surrounds and three preamp outs (Lt/Rt/Cntr) to the Krell TAS. This setup works but I refer the XLR outputs of the HTS over the RCA of the Yamaha.

Any thought? Thanks!