I would have offered around $1650 to start with.
However, good luck.
I would go look at them before mentioning money at all... I don't even email them people back, Im asking $18K for a motorcycle, and a guy emails me "whats your phone number, Im interested in your bike, will you take $15K?" that just means, after I say yes Ill take 15 for the bike you never seen or heard run, and then when he gets here he is going to start negotiations at $15K and offer me $13K... Screw that... I just ignore them emails, best bet is, "Hey, Im interested in your AV setup, is it still available, if so can you give me a call at 000-000-0000, thanks for your time, talk to you soon..."
Then when you go see them ,and they are the last items in a storage unit he is paying $100 a month for and his car has out of state plates on it and keeps answering his phone because he has a million things to do, point out 5 flaws to your friend you bring {because you never go alone} "a scratch here, oh yeah these are the old ones, whats that smell, ect"offer him $1200 cash while holding it in your hand and your friend walking back to your car saying "your crazy I wouldnt pay that they are cheaper on ebay".... He says "c'mon they were $6grand, give me $1500, and you say "OK, lets meet in the middle $1300" when the middle is really $1350 3 things can happen, he can say "nah, Ill pass", "Fine give me the $1300" or he can be good at math and say "half way is $1350" then you say "oh yeah it is, I failed math", and hand him the $1350 SOLD $1350 sweet deal good job...
Oh yeah, he wrote firm in the listing and will probably get insulted and tell you to screw, lol.... jk, Im over tired.... Good luck, I didnt look at the before, it looks like a real good deal... I would find another matching sub to that one, why not...