Tonepad Small stone distorting

[bassmeister]

I'm at the not so pleasing stage of trouble shooting, the subject of my frustration is a tonepad layout Small Stone. It sounds very good at modarate settings (sweep at midrange, color switch off), apart from that I've tried larger caps (Ca and Cb) to get rid of the common ticking problem without any results. The other problem is that about at the lower end of the sweep range, I get some distortion. Not very much, and it only appears when playing quite loud. Because of that, I figure it must be a headroom problem. The problem persists when changing R42 to get rid of the "volume drop when on" problem, and doesn't get worse in my ears.

I use a Squire Strat, a rather cheap SS amp and BC549c and BC559c (not matched, do they need to be?) transistors in the Small Stone.

Any ideas on how to get rid of the distortion? Perhaps the input amplifier needs to be re-biased when using other transistors?