I just finished an OT215 and gave it a try with my Ampeg SVT 4 pro in bridged mode. I have a Ampeg 810E, an Ampeg 410HLF, and an Ampeg Pro 15. It sounds more like the old school 8x10 - the low end was not a strong as the ported HLF cab was but MAN was it loud. Both the OT215 and the HLF are 4 ohms so I kept all amp setting the same and just swapped the cable - Now I'm wondering why I need the 8x10.
I've built 4 of the OT215 for PA use and this was the first time I plugged in a bass guitar to one of them (no HF) and I was very impressed. The 810 needs a lot more power than the OT215 and is a lot heavier but the sound was dang close.