Those look like Cheviots to me. I'd make sure the foam surround is intact as it is prone to rot after 20+ years. I own the exact speakers you are looking at and find them to be an excellent speaker. My speaker tech owns a pair and has told me that they are one of his favorite drivers. The cabinet is the weak spot for those drivers. It is narrow thus not allowing for much low bass. I haven't converted mine ot a bigger cab but will in the summer as I hear it improves the bottem end alot. I find the bass is really detailed and smooth with these speakers. Top end a touch rolled off, although you could add a super tweeter on top if so inclined. Here's a pic of mine.
What power amp are you using? I'm using a Threshold S/150 and I'll testify that this is one oof the nicest speaker/amp combos I've come across yet.
Price of the Cheviots is between $300.00-$700.00 depending on condition ect. I picked mine up for $150.00 Canadian and had a friend redo the surrounds for me. As for the D500's....they look nice but I can't respond as I've never heard a pair. Maybe someone else has.
Cheviots sold for $1000.00 US a pair in 1976 and D500's were $3800.00 in 1996. I'd offer $650.00 for the Cheviots and see what happens. Both have great resale value. The D500's are an 8" driver as opposed to a 12" driver. The Cheviots are from the mid 70's and will be prone to more problems ie: crossovers, switches ect. although mine work flawlessly.
Huh. I was gonna recommend you get them if you like the sound, then see if you could A-B them with the D500s...
More will come along, of course. If you're interested in Tannoys but want a single driver, larger than the D500s, I will probably need to sell my Tannoy System 12 DMT studio monitors soon. 96db efficiency, capable of high SPLs on 40 wpc. I tried out my RatShack meter when I first got it, using a VAC 37 wpc amp. All the LOUD I could want, hitting 115db and more on peaks, with headroom leftover. These would be in the price range of the speakers you've mentioned. The System 12s are no longer in production, being sort of replaced by the 1200s -- but the System 12 DMT cabinets are definitely more solid.
These System 12s hooked me on the Tannoy dual concentric sound, but I'm thinking of selling in order to afford a pair of System 15 DMTs.