I actually ran the Simaudio 390 with the Heimdall 2 in my system for a couple of years and found it to be a good combination. I don't think the Heimdall colored the sound which I think is a good thing. Its well built and not as expensive as some which make dubious performance claims.
I gave the Frey 2 a good audition and was prepared to buy if it brought a perceptible improvement. To my ears it did not. Any difference I could perceive was very subtle-if it was there at all-but not better. I had Hemdall 2 in my system throughout for a good while and was very happy with the performance. I auditioned other Nordost products, but thought it was more money, not significantly better performance. Just my experience, yours could differ, but it is actual experience with the gear you are working with.