Do I need a subwoofer? Which brand and model?

Make absolutely sure that you have your speaker cables connected in phase.

Also make sure that your Oppo is set up to play in 2 channel mode, and not something else like 5 channels.

Try removing the jumpers on the back of your NAD and run interconnects from the output on your Oppo going directly into the amp section of the NAD. Listen to the system and use the volume control on the Oppo.

ZD ....Pullllese.....My speakers are 5A's.....enough jest. Richard and I spoke at length about subs, and for the vast number of 5.1 systems, almost any sub (a couple of hundred dollars worth) is fine. Yes, subs are valuable in that they allow the regular speakers and amps to work without that extra load of lows. I have a separate 5.1 system in a different room than my "big" system. It is a Denon AVR X3100 with NHT floor standing speakers...with zone 2 connected to two speakers in every room of the house. It can show me what speakers are working, and generally, unless I am showing a movie with colliding railway trains, or bombs, the sub Isn't even working. (control is by Audyssy (spelling??). My sub is an Outlaw Audio set in a corner of the room, and it can shake the house when those bombs explode...and my room is quite large

A better amp will do very little to fill that room. Find the best sub you can for the money and then get another. Learn all you can about eq's, room correction and time delay.

Shugho,

Can you take an M series amp home for trial? I'd suggest you at least try it for a weekend if you can. I am not saying it works...but at least you have gone well up the NAD range to see...and if you get the right result...you can determine which NAD amp you can afford (and whether it will be enough).

I do think the sub is also definitely worth a trial...any sub.

you need a much more powerful amp. It's that simple.