Probably too late to this party but I've just read this and.... I had an SVT3-Pro for over ten years and it's deafening flat out. Here's what you need to know:-
Preamp is there for your drive and tone. Want more tube distortion? Up the gain. Put a RAT up front or a muff. The peak just means it's driving the valves into the region they aren't linear anymore. They can take it, they max out in a subtle and coloured way and then give that nice valve breakup sound, which is why you bought a tube amp in the first place. Try messing with it and see what it does. The ampeg is a tank. You won't break it. Otherwise get something with transistors and no variation of sound. Set right you can flip between clean and distorted just by how hard you play. I nearly always had the gain set around 60-80%, even with an active bass. Always have it set so at your hardest playing the peak light is at least flashing in and out. I've played Buck Rogers by Feeder without any drive pedal and got a pretty good approximation of Taka's quiet subtle clean and utterly ripped.
Set all the tone to 12 O'Clock. EQ FLAT. Always start from there. use the mid control if you need some mid scoop, but keep it subtle. Bass is, well bass-y. don't crank up the bass for volume; it will sound like elephants farting in a large hall 300 yards away. Flubby and muddy. Set to taste and sound, NOT for volume. Everything should average out around 12 o'Clock and zero dB, not all slammed fully one way or another. You can use the DI out to get a better idea of the actual preamp out sound, so stick it into an interface and check it sounds ok on some monitors.
FX Loop - stick your modulations here; we all like some flange, chorus or vibrato occasionally. Try to make it so they don't change the overall volume from off to on.
Output Section - tube gain is a voltage sag adjustment. lowering it reduces the transformer voltage supplied to the power stage tubes, meaning they saturate more quickly. This gives a warm and glowing colouration and limiting. Think Sister Sledge or The Pointer SIsters, or maybe Bootsy Collins. Turn it to max and the two valves in the power amp section stay linear and and don't saturate (yes there are valves in the hybrid power amp stage). It's in the tech material somewhere. Turn down the Tube gain to act as a soft limiter.. around 50-70% is great for all those soul and disco bass lines. Fully up for clean dry modern power, like a trace.
The Graphic should have the frequency sliders set around the middle (0dB) on average too if that wasn't clear. Can't remember if the graphic has a volume slider but put it full up if it does.
Oh and master volume - start with it about 30% up. If you do the above first then you will be able to set the tone. Use the master volume for what its' meant for. I rarely had master volume above 50% in a rock band. There are four mosfets in there doing the heavy lifting, solid state buttons of loudness. If you get bad hiss or have them anywhere near flat out you've not set the preamp correctly, you are just amplifying the noise floor of the preamp.
Really hope that helps. It really helps if you find out what each part of your signal chain actually does, and is there for. Gain staging is everything.
BTW I had the SVT3 Pro, and ampeg 4x10 HF + 1 x 15, Run in Parallel for 4ohms. tbh the 4 x 10 was enough by it's own in most cases. My basses at the time were a charvel 3B '86 through neck, a Cort C4 Limited for the funky stuff and a Sandberg VM4 (music man sound alike, better build quality :))
