I have an amplifier that has a balanced output (XLR) going to a Single ended preamplifier using XLR-RCA adapter.
Is this acceptable, any sonic degradation that may arise? If ok what is a good adapter and where can I buy it..
Need your insights..thanks a lot..
First, in terms of signal flow, the output is coming from the preamp and is going to the input of the amplifier. Second, I assume the preamp is SE and the amp is balanced. Cardas, and others, make high-quality XLR-RCA and RCA-XLR adapters but you might be better off using one cable with an XLR on one end and an RCA on the other. Most cable companies make such.

Audioparts Inc
One thing you might consider. If you use adapters, use them at the XLR end. I've found that RCA jacks can be too closely spaced to allow the XLR adapters to fit.

I prefer the adapters over the RCA to XLR IC's. Purest Audio Designs make very fine adaptors.

Why do you prefer adapters? For sound quality or convenience? I use them for the latter, only.

I ran the Cardas adapters into the balanced input on my JC1s. The sound was slightly more extended and detailed but after a few weeks I went back to all single ended. All single ended works much better in my system. Im glad I tried it but it was kind of a waste of $75.00. Tonal balance is much better single ended. I am using Vandersteen subs and single ended mod 5 x overs though. I cant help but wonder if I had balanced x overs if I might get better results. Good luck.
www.audiopartsinc.com is real good. also, www.a2Zcables.com
IMO, the answers to your first two questions are:

Yes, it is acceptable - I currently use RCA to XLR adapters because my CD player is single-ended only and my preamp is balanced only; and It Depends - as any sonic compromise resulting from the use of adapters may not be audible, depending on the quality of your equipment, synergy of the system, room setup, and (of course) your hearing.

I upgraded my preamp and had to go with adapters. I started with generic Chinese-made adapters while I considered my long-term plans. Some people have/will recommend that you have a custom RCA to XLR cable made. I considered this, but opted to buy high-quality adapters as I anticipate upgrading my CD player this year (and then I will go with a balanced cable and pitch the adapters).

As to your last question:

There are a few good quality adapters available. As previously mentioned, Purist and Cardas produce some. I was told that ARC also sells them. I bought a pair of the Cardas adapters from A-gon member Drubrew.

After they settled in they sounded much better than the generic ones I had originally.