If people say the SVT3 PRO isn't loud enough, ask them compared to what? My SVT3 PRO was loud enough for any band I played with (and some were very loud) in the 10 years I had it.
Looking at the diagrams at the back, the user manual advises the master to be full on and balance your volume on the gain. I didn't use the graphic or the buttons, tube gain at 3 o'clock and frequency on 2 and had most of the tone controls at 12 or 1 o'clock and it sounded great, full, warm and punchy through my Mesa Boogie speakers. I like clean and I'd suggest anyone wanting to dirty up the sound should get a pedal.
Biasing the transistors has been covered in Talkbass threads. This rarely needs doing but generally, unless you really know what you're doing, you're advised to get that done by a professional or risk damaging your amp. The valves are in the pre amp so rarely wear out or need replacing.