Not to hijack on the OP's subject of disappointing basses we thought we'd love, what about amps/cabinets? I'll chime my amps/cabinets: Orange & Hartke, & my first round with GK 400/800RBs.
Orange was the tiny terror bass - good tone & all, & i had read lots of reviews & people enjoying the dirtyness it gave. I tried it through an ashdown 4x10 cab & i wasn't disappointed, but nor was i excited. It was okay, the EQ wasn't spectacular. I guess it could just get the job done & sound cool too.
Hartke cabs - now i didn't ever expect to really enjoy them in the first place, coincidently this was my first time using a GK 400 or 800RB through 2 410 hartke paper/aluminium cabs. The cabs made me dislike the GK hardcore. After every song, i'd keep going back to try & get a rounder, fuller tone with an edge, but all i got was clank & thiness. I couldn't feel the bass on stage, i heard to highs it projected. Luckily years later, i got to play another 400RB through an eden cab & after a couple of initial tweaks before diving into the 2nd song, i found the tone & growl the GK is known for.
I think an amp that truly disappointed me, in the sense of perhaps "this is ground zero" for bass tone, was the SVT Classic. I'm aware that all my heroes utilized SVT's from the 60's-70's & had that classic bass tone (be it Dee Dee Ramone, Paul Simonon, Gene Simmons, & countless others), so when i played the SVT Classic, with a pbass straight in - i dialled in a good tone, i just didn't get the growl, low mid definition that my heroes had. & this is a popular amp among backlines & such, so i got used to its tone (with a 410 & 810, though prefer the 410 over the classic fridge). Go back 7-8 years & i had played through a 70's ampeg v4b & this had the ampeg growl. So much balls to the wall & great low end, low mid with enough bite for a punk rock bassist like myself

Alas, i've resorted to solid state peavey amps that get teh job done & have been the sound in my head forever. Though if i went tube - i'd go for ampeg V4b hands down.