Sorry, this kind of stuff irritates me. If someone states a price and then says "FIRM", why would you lowball him??? If you don't think the bike is worth what he's asking, walk away. If he doesn't sell it for a while because the bike is priced too high, he may drop the price or maybe just not sell the bike. Even if the price was "OBO", a reasonable offer is generally considered 10-20% below asking price, not 30%.
I guess I might be a little more sensitive to lowballers because I recently posted on CL my mint conditioin 03 Z1000 with a bunch of extras and less than 3k miles for $6k FIRM, so what's one of the first e-mails I get off craigslist? "I know you said $6K FIRM, but my offer is $4k." I almost wanted to go through the effort of meeting this a-hole so I could punch him in the nose! :mad
Bottom line: I haven't seen this bike and I don't know if it's worth $4,600 or not, but have some respect for the guy if he says "FIRM" on the price. FWIW, I don't think you could even buy a salvaged 600 from Fremont Cycle Salvage for $3,400... nevermind $2,500.