Used Audio Research Preamp Options

At the risk of showing my ignorance, I went ahead and hooked up first my CD player and then my Bluesound Node wireless DAC directly t my MC2200. I put those two separate gain controls on the amp to use and the sound quality is quite good, maybe even a tad better than with the NAD preamp in between. I left it with the Bluesound for now since most of my listening is from Tidal or some college radio stations. So assuming I have no need for two of the preamp's main functions; volume control and source switching, would my $2500 buy me better sound if spent on an preamp or elsewhere on the audio path?

May be I should start a new thread to ask a different question. But in the meantime, where I stand: I spent a good part of the day experimenting with different configurations and I've now confirmed that I can directly connect either my CD player or the Bluesound Node 2 wireless DAC to my MC2200 and get plenty - actually more than my ears can handle, of volume. Between the gain controls on the MC2200 and the volume control on the Bluesound mobile app, I have no gain/volume issues. The obvious issue is I can only use one source at a time. Should i now be looking for a passive preamp our do I just need to get so sort of a "switch" to select the desired source? If so, this begs the question why more folks take this route and spend mega bucks on a (active) preamp. Does a "good" preamp add to sound quality - soundstaging, imaging, etc., above a direct connection? Sorry if I'm asking the obvious but I was caught a bit off-guard discovering that I could even run my system without a preamp. Thanks as always for your great input.

I had previously owned an Audio Research SP-3 and LS-3 and earlier this year decided to upgrade preamps. After considering some of the excellent suggested used ARC preamps mentioned in the above responses, I decided to pick a new ARC LS-17SE on closeout from an authorized ARC dealer for $2795.00. Overall, I felt this was the best deal for me as the preamp listed for $5995 and was new in a sealed box from a well reviewed and authorized dealer. This unit was discontinued in 2015 as part of ARC's ever changing product lineup.
I thought although there are some good buys available for some used ARC preamps, issues such as lack of warranty, possibility that tubes would need replacement and that given a need to replace caps after (in some cases) within 15-20 yrs., I felt a new unit was the way to go.
Since these units are still available at this price, you may wish to consider that option as well. I have been very satisfied with this unit's build quality and sonic performance and would not hesitate to recommend it as a best buy in the price range that you are looking for.

I agree with nmolar above. I have the same pre-amp, and for my taste, and with the gear that I have, it is a wonderful pre-amp at it's price point.