@kaizen28 I just deleted a lengthy post to share two thoughts, maybe three (Edit: or four).
1. You should purchase a new AV receiver, and it doesn’t have to have a preout, which will save you a lot of money. Use the Naim RCA sub/preouts as an AUX input to the AV receiver . . . if you also
2. Make sure your new receiver has the equivalent of Sony’s ‘Pure Direct’ mode that can be easily switched on/off (Sony puts a button on the remote). The Pure Direct feature and its imitators (or Sony imitated others, I’m not sure) allows the original signal through without any additional processing, stereo is stereo, not multichannel stereo. On my system it really cleans up the signal; output to the center, satellite, and subwoofer speakers are dropped completely out.
3. Add one or more subs, driven by the AV receiver. At least one manufacturer makes speakers that accept both the speaker output of an amp passively, and the sub-out signal into an integral plate amp actively. This would save you from having to add additional boxes to get Low Frequency Effects (LFE) for life-like movie sound. You could hold off on this development if . . .
4. You find you prefer how your speakers sound as driven by the Naim, in which case my suggestion won’t work for you. My suggestion depends on preference for the speakers driven by the AV receiver. In my case, the much newer, much cheaper, Sony AV receiver bettered my older McIntosh stereo amps configured as monoblocks, even when driven by a top-of-line Rogue tube preamp, but only in Pure Direct mode. Not what I expected, but I’m learning to trust my ears. Your mileage may vary.
Note: Using the Naim as a preamp would mean having two volume controls in series. I have found, in this situation, it is best to ‘fix’ the first volume control at maximum (100%) and control the speakers only with the last volume control in the chain, in this case, the AV receiver.
As stated at the top, there are many ways to skin this cat (AV receiver + 2 channel sound), and many, if not most, of the posters to this thread have far more experience and knowledge than I do. I’m simply sharing a strategy and its components that have worked for me.