I cannot be certain, but my vote would still go for the Threshold 550e. I did not have time to listen to a large number of deep bass pieces and in sequential listening - one week v one week - one's sound memories can become faulty. Nonetheless, my view would give the deeper + tighter vote to the Threshold.
Mac 352 v s550e Threshold amp
In an expired thread, I mentioned the opportunity to compare the Mac 352 and the Pass-designed s550e amps. Finally, this has been done. I am opting to purchase the Threshold at a price of about $2300 after netting out the value of a pair of top Esoteric cables and a dealer guarantee of five years. The Threshold was, for a time, a high end demo unit and is mint. I found the Mac to be very good but, to my aging ears, not as tight in the bass and linked to just a bit of coloration in the midrange. Nothing really bad and someone else might hear it differenty. However, even with an aggressively priced Mac 352, the price difference with the Threshold was significant and I have known and trusted the dealer for a long time. It will be replacing a Threshold s300 which I plan to retain, at least for now. So, maybe I have a Threshold bias. Do not misunderstand, the Mac 352 is a good amp. That is just how it came out for me. Others who have heard both or either may wish to comment as well. With thanks to previous respondents.