It seems the tubes that came with the thing were at least having problems (probably the reason the seller sold the RKV, given how hard the PCL 805 tubes are to find, as I found out), even up till now. Thinking I found the problem, I tried swapping back to the old tubes, as to not waste them if they still worked...but nope, the funky warped sound came back using the Senns. So the old tubes are probably bad period, with no hope for them. And yet, if I turn the amp off and on again, the sound will return back to normal, if for a brief period only.
By warped sound, I mean the sound will gradually start dimming, and the soundstage will slowly narrow into pratically nothing. Not sure if you would call this clipping, but it seems to be a problem with the tubes, rather then the amp itself.