Keep them. They're worth more to you than anyone else.
IIRC, Proton is effectively re-badged NAD. The most recent amp sale I could find in a cursory eBay search suggests the amp is worth about $400; the last preamp sale suggests it sold for $300.
If you're serious about selling them, this is a good site, as is eBay, etc. Assuming they both work properly, list them as a pair for $700 and see how it goes.
IIRC, Proton is effectively re-badged NAD. The most recent amp sale I could find in a cursory eBay search suggests the amp is worth about $400; the last preamp sale suggests it sold for $300.
If you're serious about selling them, this is a good site, as is eBay, etc. Assuming they both work properly, list them as a pair for $700 and see how it goes.