I've just (2 days ago) got a used SVT 3 Pro to use with a BF Compact or Compact/Midget setup. I will record my thoughts elsewhere, but my top advice would be to [b]buy used[/b]. A new one (far East manufactured) will cost you about £1,000, whereas used they go for £350-£450 here and on eBay, and many of those available are USA-made (fwiw).
Yes you need to turn the master up high to get the full beans, but you can add more volume by using the graphic's volume control.
One issue to be aware of is that the bias of the output transistors can drift, resulting in loss of power and/or crossover distortion. Setting the bias requires a tech with the right gear - if you just get in there and twiddle the trimpot you can fry all 8 MOSFETS in seconds. Ampeg have fitted a cheapo trimpot, so you may want to budget for getting a multi-turn trim fitted.
There are a couple of threads on TalkBass about this - just put the following into Google: [b]site:talkbass.com svt 3 pro bias[/b]
Having studied the schematics of both the SVT 3 Pro and the Streamliner 900 (which I also own), the overall concept of the preamp is very similar:
1st valve - buffer stage
2nd valve - passive treble/bass
3rd valve - active mids with selectable centre frequency
I suggest you also look into the Genz Benz Streamliners. Although they lack the extra tone-shaping gizmos of the Ampeg pre, they are currently excellent value if there are any left (600 @ £399, 900 @ £499), light, loud, and compact.