Running your amp speaker out --> Ultra-G --> Weber Mass can be a good chain and pretty inexpensive.
I did some objective testing with a Bryston 2B a few months back and found that if you use the Weber Mass as a dummy load and tap the line out on the Ultra-G you get a virtually flat response. Tapping the WeberMass rolls off the lows quite a bit, which lead me to my "runs on the bright side" comment.
Here's a graph of my test. The pink line is the original signal (pink noise). The yellow line is the Weber Mini Mass (treble on zero). The orange line which overlaps the pink one, is the Behringer Ultra-G (G100) into the Weber Mass (as a dummy load only):
Based on comments in this thread Weber may make you a Mass without a line out which should make it even cheaper.
Unfortunately, different tube amps react differently to different loads. I've done some testing using various tube amps of that and gotten mixed results. Here's a graph of an Emery Sound Microbaby with a 6V6 into a Jensen C12K (orange), a Weber Mass (yellow), and a Tube Town Tonehound (red):
The Tonehoud, near as I can tell, is just a couple of midrange speakers in a box. The missing low-end bump you could make up with EQ or mic placement. The high end roll off, I'm not so sure about. You human -- I mean US human are far more sensitive in this region. So, the jury's still out I guess.