Which ARC amp to pair with my ARC SP9 MKIII

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).

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

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.