Who out there knows about Audio Research Pre's???

Out of all the the used tube pre's which one offers most bang for the buck?Also if it is the SP6 how much odes the A version differ from B-E?I notice a number of LS22's which seem more recent in vintage and list for $4K going for $1500 and they look form back to be fully balanced (though looks/XLR's can be decieving).How old are they and why do they go so cheap.The LS25II seems to be most recent and uses the 6H30 (??) "Super Tube" like BAT.Seems a bit to rich for my blood.But want something to replace my Muse at some point to match with a Mesa Baron and about 10 different spaekrs I will own over next few years.