I'm not aware of any metal albums that feature "only" bass (that wouldn't really be metal anyway), but here are some recommendations -
Spastic Ink. Their first album Ink Complete was purely instrumental. Their second, Ink Compatible has some vocals but also features a far more impressive bass lineup - Riot's Pete Perez, Cynic's Sean Malone, Attention Deficit's Michael Manring, and Watchtower's Doug Keyser. Both are highly recommended, and the latter especially has some absolutely eargasmic bass interplay.
Also not instrumental, but definitely get your hands on Furioso from Pavor... some of the most unbelievable bass work ever put on a metal record.
!T.O.O.H.!'s Order and Punishment has impressive basswork.
The self-titled Aghora album would be well worth your time to check out.
Cynic's Focus and Atheist's Unquestionable Presence. Musts for any metal collection (IMO) and godly bass work to boot.
Alarum's Eventuality - Cynic/Atheist worship band.
Coprofago's Unorthodox Creative Criteria - Cynic/Meshuggah worship band.
Intronaut's Void - sludgy metal with a very lush and organic bass presence.
Control Denied's The Fragile Art of Existence - a side project of Chuck Schuldiner with Steve DiGiorgio on bass, I believe.
Quo Vadis - Defiant Imagination - more DiGiorgio goodness.
Before somebody jumps all over me, I know that none of the above are instrumental. However, they will most definitely please anybody looking for prominent bass work in metal.
Here's some more stuff that is purely instrumental -
Behold... the Arctopus - Nano-Nucleonic Cyborg Summoning - seventeen minutes of absurdly technical insanity with the Warr Guitar as the centerpiece. Impossible bass heroics.
Gordian Knot - Gordian Knot and Emergent - project of Cynic's Sean Malone, so of course the bass work is great... but it isn't really metal.
At War With Self - Torn Between Dimensions - sort of like Gordian Knot, with Michael Manring on bass.
Dysrhythmia - Barriers and Passages and Pretest - again, I don't really consider these guys metal, but they're close enough and the bass work is great.
You can also check out drone/doom, I suppose. Really earthshaking stuff like Electric Wizard and whatnot. I'm not really into that as it's way too slow for me, but it could be what you're looking for?
I'm sure I'm forgetting a bunch, but those should satisfy any bass freak for a while.