Scotch, I can chime in there.
I went direct with it using the upright and using the U-Bass. The thing is really cool for sure. The 4 MegOhm input impedance is a neat sweet spot for upright. I just had a custom pedal made with 5 Megohms and the upright pickup really likes that range.
Unfortunately the price tag really doesn't justify that application IMO. The tube sounds great with the 51 P-bass and I found a sweet spot on the drag control for making that bass sound old school, with looser lows. I'm sure it would speak great on tape. It sounded good through my amp. And saturating the tube was not a desirable tone IMO. It sounded like more of a solid state thing.
It did not hold a candle to the U-Bass through the F-Deck. I have yet to try something that does. I believe the Radial PZ-DI comes close. Chad had more rime with that box. If the F-Deck had a smooth filter for the highs and a direct out that would take the cake. But it is my favorite front end and filter for lows. I am working with custom builders to take care of the upper filters.
Sorry to disappoint you, because the thing is really cool. It is a doubler's answer to the avalons and such. But for way less than that I bought a Genz Benz Shuttle. It has a lot more on it, is tube, and the clean sound of the DI is almost identical if not better.