chimmycham, I own both of those amps and between the two I think I enjoy the Vali a little bit more.
I can't put that down to any macroscopic difference: to me it's not the case that the Vali is "warmer" or that the M2U is "more detailed." It's much more subliminal. For some subliminal reason the Vali pleases me a little bit more than the M2U does.
That said, I would suggest that the M2U is the somewhat safer choice:
1. It has a gain switch.
2. There is no chance of tube microphonics.
The gain switch is relevant because it will let you use the amp with a wider range of 'phones. IIRC the HD598 is a pretty efficient headphone and you may find that the M2U's low gain setting of 1.5 gives you a nice range of volume adjustment without straying quickly into too-loud territory. (My HD598s are in storage so I can't test this for you I'm afraid.) The Vali's gain is fixed at 4, which isn't particularly high but may give you less of the volume knob's range of motion to actually work with.
(I use my Vali with an ODAC, which has an adjustable output level, making this a non-issue for me.)
Tube microphonics: nice as the Vali sounds, some users have reported ringing when the unit is tapped or even when it's sitting on a desk while they're typing. I do not have that problem, which may be because my unit is a new one (early 2015) built after they started adding some extra cushioning around the tubes. But maybe I just got lucky with this particular unit. I don't have any good way of knowing.
As for the DACs, it depends what you're looking for, and there I can't advise you. The Loki does one thing only: DSD audio. If all your music is DSD it's a good choice, but that would make the Modi irrelevant. And if your music isn't DSD then the Loki's irrelevant. If you have a mix of the two then buying both would be appropriate.