Will a outboard speed controller improve sound?

short answer is yes. is your LP12 a valhalla? About two years ago I picked up a mid-90s valhalla that i upgraded with a mose/hercules PSU. It functions exactly like a lingo and can either replace the Vallaha board in the 'table or be used in a separate case with an umbilical cord, no drilling or new holes required bascially plug-n-play. you get a new two-speed switch so you can change speed on the fly. it comes over from the far east but shouldnt put you off, its very well made. you need to figure out if you have a 50 hz or 60 hz motor. Lp12 basic came with a 60 hz motor, valhallas came with 50 hz motors.
http://www.ebay.com/itm/The-Mose-He..._Accessories&hash=item5af06d45c6#ht_587wt_899

I owned a high-spec LP12 back in the day with a lingo MkI and imo its not worth seeking out a used lingo mkI at nearly twice the cost of the new mose.

has your valhalla been re-capped? theres an electrolitic on the board that fails with time. the speed on my table was drifting with the stock PSU, so the improvment was obvious. the mose is not a lingo copy but more like a redesigned valhalla that allows speed changes electronically at the push of a button. heres the link to buy directly from the mfr in HK: http://www.herculespsu.com/