imo - the point of getting the acm mod done (and i have an acm P2) is so that you can run the box as a standalone. basically, if i understand what was done correctly, the preamp stage (the analog stage) has been rebuilt by doug, lowering the noise floor, and likely making the response quicker, and generally improving the signal quality. if you are going to use an external preamp, then you might just want the basic mod which rebuilds the rca line-in stage (not the xlr-in) which is what you would be using with an external preamp. if you have an external preamp AND adc that you are using (like a V3), then you might not need a modification at all, 'cos you would just be using the p2 as a bit bucket...
hth