"Prior ARC stuff had cheap caps, cheap RCAs..."
Audio Research never used cheap parts. I have owned many Audio Research preamps including the SP-3A-1, SP-8, SP-8MKII, SP-9, LS-1, LS-2, SP-14, SP-11MKII and SP-15. All of these preamps were made with high quality parts.
If I remember right the SP-15 is actually a solid state line stage with a hybrid phono stage. The SP-15 is all about phono and when it was manufactured it was one of the best phono preamps money could buy with plenty of gain and flexability for whatever cartridge you wanted to use.
Keep in mind these hybrid preamps need to be left on to sound their best. It takes them 3 days to come around. If my memory serves me well the 3 tubes in the phono stage are rated to last 10,000 hours and they are not expensive tubes. It is well worth leaving the preamp on for the improvement in sound.
The gold Tiffany style RCAs used on the SP-15 were also used on the SP-11 which was introduced 1985 and the SP-9 in 1987. Audio Research amplifiers were also using the Tiffany RCAs before the SP-15. Not that the previous RCAs were bad. They were very good also, it's just that the Tiffanys were not available.
All of the Audio Research preamps are good in their own way, but do not try to evaluate one of the hybrid or solid state preamps without sufficient warm up or you will be disappointed. After a good warm up they become a different animal.