Didn't want to buy a $1,000 subwoofer setup, only to discover that I must also add a buffer, so I have been putting off any purchase. Then a few days ago, I picked up a ML Dynamo 300 sub for cheap, mostly to see how it's low input impedance (10K ohms, same as my Aleph 2 amps) would work with the amps/Martin Logan electrostatic speakers. Much to my surprise, the Dynamo sub and Aleph amps work very well with the Tortuga - I don't detect any negative change with the sub in the system. To make sure, I turned on the sub, but set the volume to "0" and had a listen. Then I disconnected the sub and had a listen - no change, so the sub's low impedance is not affecting the SQ of the Aleph/Martin Logan setup. With the sub turned on, there's not much difference in the bottom end, but that was expected as the sub has an 8" woofer, as do my speakers.
I will probably buy the Dynamo 800X, which has a slightly lower input impedance (8.3K ohms), but I think it will be compatible, too, with the Aleph amps.