just picked up a vintage blackline svt and roadcase for $1,000 after a multi-year search and much patience. it has 4 year old preamp tubes and 1 year old 6550s. it is well worn but works great. mfg label says "made by ampeg". questions: would that mean it is a pre maganavox-era example? possible to positively ascertain the actual year of mfg? any issue with using an 8 ohm 810 cab other than reduced power output? anyone know of a link to user or service manuals? thank you!