I loved my ZH270 and still miss it, but I couldn't afford to keep it after I bought a Siegfried...
In addition to the many things you've already mentioned, I found that the ZH270 was sensitive to the type of feet it sat on. My best results were leave out the cones and spikes and use the standard plastic feet on hard, heavy surface.
Cables do matter! The Siegfried is merciless in revealing colorations in the input cables...more so than the ZH270. I get nice neutral sound from Kimber KS-1011. I found that my two "silver" cables had undesirable "zip" in the sound on the Berning amps. Also, I found that monowiring gave a more pleasing sound than biwiring. I use the same biwire cable as before, but I connect the the high and low end cables together again at the speaker end. This gives me a much smoother high end. Your experience will surely vary, but it might be worth a try.
Are you using any power conditioners before the Berning? If so, try without them. I get noticibly better results with the PS Audio Ultimate Outlet.
After buying my Siegfried, I horsed around with substituting NOS tubes of various types. David Berning warned me that it can be a "real adventure" in finding tonal balance...and he was right! I am now back to the original tubes, including the ordinary Sovtek 6SN7s, and all is well. I was able to get best results by choosing different cables and repositioning my speakers.
Do you get the same tonal balance from both speakers? Try right and left alone with a mono signal from your preamp. If they are strikingly different, try swapping right and left speakers to rule out differences in tone to speaker placement. If the problem is in one channel, then you may indeed have a problem with your amp.
In addition to the many things you've already mentioned, I found that the ZH270 was sensitive to the type of feet it sat on. My best results were leave out the cones and spikes and use the standard plastic feet on hard, heavy surface.
Cables do matter! The Siegfried is merciless in revealing colorations in the input cables...more so than the ZH270. I get nice neutral sound from Kimber KS-1011. I found that my two "silver" cables had undesirable "zip" in the sound on the Berning amps. Also, I found that monowiring gave a more pleasing sound than biwiring. I use the same biwire cable as before, but I connect the the high and low end cables together again at the speaker end. This gives me a much smoother high end. Your experience will surely vary, but it might be worth a try.
Are you using any power conditioners before the Berning? If so, try without them. I get noticibly better results with the PS Audio Ultimate Outlet.
After buying my Siegfried, I horsed around with substituting NOS tubes of various types. David Berning warned me that it can be a "real adventure" in finding tonal balance...and he was right! I am now back to the original tubes, including the ordinary Sovtek 6SN7s, and all is well. I was able to get best results by choosing different cables and repositioning my speakers.
Do you get the same tonal balance from both speakers? Try right and left alone with a mono signal from your preamp. If they are strikingly different, try swapping right and left speakers to rule out differences in tone to speaker placement. If the problem is in one channel, then you may indeed have a problem with your amp.