Great to know you are getting the new toy soon! Looking forward to your review of the installed product.
I'm very sorry you have had no input regarding your concerns. Until someone with actual experience with the AVA comes along, I can only comment on the last point:
...Is it even worth it to send the output to IA18 for better SQ, or should I just use the internal amp on the HD-AVA? The IA-18 looks like an amazing product, so I feel bad not utilizing it.
If you already have the IA18 (which I agree, looks great from the reviews), I believe it would be a fun experiment to try them both and just decide which is to your preference?
If you prefer the NuPrime's stage, you can sell the IA. If, on the other hand, the AVA used as a PRE and the IA as an AMP yields better results, you already have all the hardware to produce great sound! I actually envy you a little
Let us know how it turns out, best regards,