The Monitor Audios are great sounding speakers for the price point. Perhaps not the most revealing in the upper treble, but their pitch, tone and timing can be quite good when matched with the right electronics.
I have some MA Apex 40's which I believe use the same tweeter as the Silver S2's except in a MTM arrangement. These MAs are in the front end of an HT system which is driven by an Arcam AVR-400, and in either 5.1 or 2channel the amp-speaker combination works extremely well. With a decent analog or digital front end the soundstage is well presented and male and female voices are particularly compelling.
Due to my experience, I would think that Samac's suggestion of the Arcam A39 is solid. Perhaps any quality amp that excels in the areas of PRAT and purity of tone would match well with the Silver S2's, making the usual English designed suspects or similar pertinent.
FWIW, I tend to fall in the higher investment in electronics side, but the codependency of gear, ear and room make any strict rules
Almost Irrelevant.
note 1: MA Silver S2s have a fairly big woofer, so do not under power. Your NAD should generally be fine on that count.
note 2: advice to point Silver S2 tweeter at your listening position is spot on and will reward.
note 3: an NAD weak point is metal jumper bars in the back connecting the preamp and amp sections. Try replacing these with decent reasonably priced 0.5m interconnect like Audioquest diamondback or king cobra or similar from another brand (but not AQ dedicated preamp jumpers, not that great). This should yield a noticeable improvement. You could experiment with silver and copper wires if you have both in hand. A cheap upgrade while you sort out move to different electronics.
note 4: I doubt that the preamp section in the OPPO is better than the NAD, but at least it would take the sucky stock jumpers out of the signal path (see note 3 above). If you try this, compare with IC in NAD Pre-amp signal path to make sure it is apples to apples comparison.
