Short-term solution:
Get a Boss OD-1 pedal. This is a booster that is intended to create distorion by overdriving the input of the amp. In your case it will simply make up for the attenuation in your preamp. The pedal output can be adjusted over a wide range to dial-in just the amount of drive you need.

Long-term solution:
Haunt eBay, pawnshops, and and the used gear sections of as many music stores as you can get to. Your quest is to find a late-70's Ampeg VT-40. This is a 60 Watt 4x10" combo amp with a Master Volume. I bought my '76 as a flood-damaged salvage job, so I ended up building a new cab for it (1-15"). My point is, this amp sounds good enough that I was happy to put that kind of effort into rebuilding it. The adjustable input sensitivity allows you to control whether the preamp stays clean or gets overdriven. The pair of 7027A output tubes have a very smooth distortion characteristic that lets you sound good at fairly low volumes. Not 'practice-in-your-apartment-at-3:00-AM" kind of low volume, but certainly a lot less than the 'Voice OF God' 70 Watts available at full output. My non-master volume, 120 Watt, VT-22 isn't noticeably louder - it's just that it stays clean until it's loud enough to make your ears bleed.