MOTM

I once owned 2 ARP 2600s, both with grey panels, but the two filters 
sounded different, I'd always wondered why.  I purchased one new 
sometime around 1977, the other I got used roughly 6 months later.

I would agree that 90% of the ARP sound comes from the filter, but I 
would also argue that the oscillators played a part in the sound as 
well.  If you just listened to the oscillators without the filter, 
they sounded somewhat thin as opposed to the fat Moog sound.  

The ARP definitely had a somewhat nasal quality as was pointed out 
by someone else.  Also the keyboard was a pain to use.  The feel 
wasn't good and if you tried to play it quickly sometimes an 
occasional note would sound flat, as if proper contact were not  
being made.