Need help picking a preamp.

I'm a long time listener to music and am trying to slowly upgrade to a better quality sound. Looking for a preamp in the 500-$600.00 range but am willing to go to 700-$800.00 range if the extra money would equate to a difference my ears would hear. Do not know which is better (SS or Tube) in my price range and with my setup listed.I assume used in the way to go.(Jazz, Classic Rock, Rock)

Nad C720BEE
Yamaha Av RX-V530
Onkyo disc
PSB Image T65
PSB Image S50
PSB Image C60
Yamaha Sub SW 90
There are several 1990's or early 2000's Classe preamps that you can get used, in those price ranges, assuming you don't need a phono stage (I can't tell for sure whether or not you do based on a quick look at your component list). It may be possible to find one in that price range that has a phono stage, as well. Do a search for "Classe" at Audiogon under solid state preamps, and you'll probably find several available. They are reputed (rightfully, imo) to have a tubelike sound while having the advantages of being solid state. Their service and customer support is also reputed to be top notch.

-- Al
I've had many. My ultimate is an MFA Magus. More reasonable is an Anthem TLP for solid state or AR 6C for tubed. They both beat my Classe 30 hands down. The Anthem has a built in tuner, the AR a phono pre. Probably $550 and $800 respectively.
Classe is an excellent way to go. I'd agree with Almarg.

A Musical Fidelity A3CR pre-amp used would be another good option close to that range particularly if phono is desired.

Those are the ones in that price range I actually considered recently. I ended splurging for an Audio Research sp16 tube pre-amp that cost a good bit more in the end.
There's a TAD 150 signature posted here for $700 - which is a good price on a great tube preamp. I don't know the seller, but own one myself.
If a tube pre-amp piques your interest, Juicy Music pre-amps are also highly regarded by many and often come up for sale here for under $1000. These were on my recent short list as well.
I think it would be a waste of money -- just not enough dollars there to make an improvement. You can definitely get a "different" sound at that price. I would wait until you have a much larger budget and an idea of what power amp you'll end up with.

I assume you would run the preamp into the Main Amp In jacks on the NAD. If so, then you need to be aware of the possible impedance mismatch issue with a tube preamp. I don't know the input impedance of the NAD's amp section. Tube preamps can have a high output impedance that require an associated amp to have a very high input impedance.
How much more of a increase in $'s would I have to have to hear a difference. Would I also need a new amp, if so, how much more of an amp?
Mapletree tube preamp is awesome little bugger for the money.
Around $800 new.
It will serve you for many,many years and it is a pleasure to listen to.
I know few guys who own this preamp with systems FAR more expensive then yours. Juicy Music Peachtree is also great preamp if you can find it used in your budget. Integrated amp is another option which you can consider.
It might be a little over your budget tho. There is one you might really like up for sell at great price. The seller won't ship but it doesn't hurt to ask - DK Design vs 1 mk ii Integrated amp.

Oh yeah........everything will change the sound.
Speakers and amp is a good starting point and the most important factor to successful system.

Gary, honestly you have good speakers and the NAD integrated is well suited to them. One option I would be comfortable replacing the NAD with is a Parasound Halo P3 ($850) preamp and an ATI AT2002 ($1.5K) balanced amp.

Since you've got a multichannel system, have you considered upgrading the AV receiver or going to a separate pre/pro and doing away with the NAD? With either option you could get Audyssey room correction and that would likely be a better investment.
Eastern Electric Mini-Max. No comparison.