Since @Roadog66 resurrected this thread after 4.5 years, I feel that I now have experience to address this thread better for anyone coming upon it via search.
I've owned these Ibanez basses: GSRM20 Mikro, SRMD200 Mezzo, SR500, BTB33 Volo, SRSC805 Cerro, SR1305E, and SR1400E. I still own all except the SR500.
As mentioned, there's a general trend toward better fit and finish, quality control, and pickups/preamps as the price goes up with the Ibanez models. The higher-numbered Standard basses are a little better finished than the lowered-numbered ones. The Workshop basses are similar to the highest-numbered standard basses. I'd say there is a definite step up for the Premium models. The overall quality is excellent on those, and they have stiffening rods in the necks, which are not found on most of the Standard models. Given how slender the necks are on SRs and BTBs, the rods are a feature worth paying for. I also really like the Nordstrand Big Singles that come with most Premiums. The Ibanez preamp is good, in my opinion, and the passive tone control is a really useful feature.
If I were shopping for a new bass--and I am--and I wasn't looking something "weird" like headless or fanned frets, I'd take a hard look at the new-for-2020 SR110*B models. They are the cheapest Premiums. They give you the build quality, pickups, and electronics of the Premium line without having to pay for exotic woods (which look cool, but don't contribute to playability or tone. I'm thinking of picking up an SR1100B this year.
I'm also looking hard at the EHB1505MS, but that's a different story.