Which ARC amp to pair with my ARC SP9 MKIII

A suggestion: if you are close to a decision on an older ARC amp, you may want to check with ARC regarding the age/condition of the power supply caps. I have heard that in some older ARC amps, the power supply caps may need replacing, which is a very expensive repair. I am aware of people having to replace caps in D125's and M100's. Also, the sonic degradation due to caps going bad may be gradual in nature and hard to detect.

often you will find a D-115 at audiogon.it a great amp.
was often paired with the SP-10 pre amp (all tube) very nice

I'm not sure how much power you need and how big your room is, but the Classic 60 or V-70 might be a good match for the sound you're after, hybrid amps like your preamp. Seems more powerful than its rating, too. I also saw a pair of VTM 120's on the 'Gon for a little over your budget which probably won't be quite as detailed but should sound more fleshed out and tube-like (if the difference between the Classic series and the VT series in the lower power models is the same as in the Classic 150/VT150).