"i'm guessing its just the recording clipping but can't say for sure. plus it doesn't sound like the recording is brickwalling at all. however, i had the ua-5 knobs past 3 o'clock...and i've read bad things about that before."
I have never had a problem with cranking up my warm mod ua-5. Wonder if some have different parts than others? I was doing some test recording at home and had the warm mod cranked up at around 5 o'clock without any problems.