Again, I'm forced to work from the opinions of others, but I suspect that only a chosen few will be in a position to compare all 3:
- if you can afford the SR-71 Blackbird, get it
- if you can afford the P-51 Mustang, get it
I did find an SR-71 in Singapore recently but the asking price was hilariously high - I think it was just there to drag people in the door. By contrast, the D4 I purchased here in Thailand yesterday came in at about the same price I would have paid in Aussie dollars with shipping added, and that is well under the landed price of one of the RSA amps. If you want something that sounds good straight out of the box, I'd look further afield than Ray's amps. I know it has been a huge FOTM, but thus far I am very happy with mine - as always, if you use sensitive IEMs or have preferences which dont align with a particular amo, this may or may not be your best option.