My vote for the ranking is nearly identical to Ronnie's, however I would put the DSP7000s and DSP5500s in nearly the same position as I felt there was very little difference between the two. Since you are considering buying second-hand and mentioned that you are keen to maximize your investment I think you would be better with 5500s over the 7000s since the price difference between the two on the used market is substantial.

For what it's worth, I would go so far to say that the DSP6000s are the best speaker value I've heard considering their current used price. A trio of 18-bit speakers was advertised in NYC several months ago with a 561 for USD $4K! When you consider that you are getting high-quality drivers, amps, and cabinets, in addition to a speaker that was class A rated for many years, I think it's a no brainer and an absolute steal!

John

Theater: 861v4_LPS, 818v3, HD621, Oppo 203, Lumagen Pro, DSP8000SE, DSP5500C, SW1600x2, DSP5500, Epson 6040UB, PS4
Living room: 861v4_LPS, HD621, DSP8000SE, DSP5200C, Oppo 203, Lumagen 2143, 818v3, Sooloos/MC600 Bluesound Node
Bedroom: M80
office: G68D, DSP6000
Garage: DSP5000
Storage: M33s, DSP33s, DSP5k, 501, 518, M1500, G98DH, SW1600