I am not sure why the walnut finish is so important, but buying a sub that performs well would be my first priority and then add your own finish and get what you really want I would think adding some walnut veneer would not be that difficult. Time consuming maybe but not difficult. The SVS subs offer a lot of bang for the buck and their customer service is said to be pretty darn good.
Sub suggestions please
I just got a pair of JBL L100 Classics (I have seen that they are controversial here, but I love the look, and am impressed with the sound...I'm a happy camper). They are connected to the Cambridge CXA61, and I run a Technics SL1600 with a Grado Prestige Green3 cart, and a Cambridge Alva Duo preamp, and Cambridge CD transport (CXC V2). My sub is a Klipsch Reference Series 12", which I like, but it doesn't aesthetically work with the JBLs. I'm considering the REL Classic 98, Sonus Faber Gravis II, or the SVS SB-3000 (I was told I could get it in a walnut veneer, but haven't actually seen that. Walnut is important in this case). Any seconds on one of those options? Any alternates I should consider at $1,500 or less (I am perfectly satisfied with pre-owned)?