if the 90 LP has the stock pots, they may be 300k which will lose highs when turned down. You probably k ow all this but others may not. At least those pots need to be almost off to clean up due to poor taper. The last 10 years find Gibson’s with better tapers. The handful of Gibson’s (and fenders) I have now all clean up closer to 5-6 with with little loudness reduction on any plexi type including the sv20. Then from 1-6 volume goes down. Saying that, I crank amps well into PA distortion and don’t use dirt pedals... never needed another channel. I found the exact same regarding the 2061x. I had one and also a 1969 2022t both 20 watt with el84s. They didn’t sound any thing like a regular 50 or 100 watt plexi compared to these sv20s. I gotta say, if I could only have one production line amp from the last decade, the SV20H with an attenuator would always win. Glad you’re digging it so far.