Just to add I asked over at Sewatt and the output transformer on the stock Valve Special is bad. It is not an air gaped core as an SE amp transformer should be so replacing it would go a long way to helping the tone of the amp. Also you could get a transformer with multiple output taps so you would not be stuck with just 8 ohms output impedance.
But to go along with above trying the amp through other speakers would be the best first thing to try, just use the head section and play through some cabinets if you can. I know the Bugera V5 sounds dark and that is due to the stock speaker, I was surprised that an 8 inch speaker would lack highs but it does.