This may be a well known tweak for any amp but I am so impressed with the result I thought it worth sharing.
Because my pre-amp has no balance control and a very high output I wanted to have an input volume pot fitted to each channel of my Radford STA15. I decided I could live with the output impedance being fixed at 8 ohms and therefore could remove the selector switches and install the pots in their place. David Coe at AD Audio carefully selected a pair of pots and did the work.
Having got it back, I am amazed that a change carried out for the sake of convenience and usability has produced such a dramatic improvement in sound quality. Whilst it was no mean performer before, there is now an improved clarity across the range with particular improvement in the bass. It doesn't go any deeper, but what there is seems stronger and more focussed.
Of course should I need to, the output impedance can still be changed but not just by turning a knob.
Be interested in knowing if anybody else has tried this.