I should have mentioned the X-Sabre from Matrix earlier when this discussion came up. I haven't heard it side-by-side with the M51, but my impression having listened to both is that the M51 may be slightly warmer sounding. The X-Sabre is musical, but very clean and smooth. It can sound harsh if a recording is harsh, but to me that's how it should be - I don't want stuff too smoothed out. It's a really nice compliment with my various tube and SS amps combined with the T1s. Soundstage is excellent as is detail retrieval.
I can't thank you enough for your help, Loquah. I will add the X-Sabre to my list of DACs to check out!