to gain fully integrated use of an alpine processor, you need an alpine HU. it won't be anywhere near as elegant a setup with your pioneer HU. it means you no longer use the volume control on the pioneer; instead, you set it to a preset level and the HU functions like a 'source' unit, just like a home hifi CD player or tuner. you then mount the hardwired remote for the PXA somewhere on the dash, and use its buttons for volume. i think this is rather cumbersome.
if you want some advanced processing, i'd recommend one of alpine's high-end HUs, like the CDA-9855R for RRP$799. otherwise, buy a cheaper alpine HU, and add a current alpine processor, the PXA-H701 + controller for around $1000.
the PXA-H600 is quite limited in function compared with the current crop of alpine processors or even HUs like the 9855R. this HU has more features than the H600.
from your perspective, with a pioneer HU, i wouldn't pay more than $50 for the H600. also ensure it's complete with the hardwired remote control panel and powerwiring harness.