My experience is somewhat limited, due to being a guitarist primarily and having only 30 minutes or so with the 400 and 700. But here's what I thought ...
SR300M (newer five pot model with N2 pups) - felt just like my SR370M in almost every way, a little different in the weight, even though I think it's the same wood. But still the lightest bass I've played since my SRX390 and SRX2EX. Neck is fast as hell and easy for guitarists to transition to. Playing way up the neck is very easy yet I never feel 'crowded' down near the nut despite the narrow width. The frets were not dressed well past the neck joint, just like 370. I love the new CAP EXF N2 pickups, they sound just like my older Ibanez pups (that have too much radius), but all the strings are equal in tone and volume. Ibanez listened to complaints I reckon. The add copy I saw even mentioned keeping all the strings at equal volume with the EFX N2s. The radius is very noticeably reduced and sounds much better as a result. I can get a crystal clear hi-fi tone for slapping, roll the treble pot back for a smooth punchy tone, or add/subtract a touch of bass for different rooms. The preamp gets little respect on TB, but I like it just fine with the new pups.
SR370M (older 4 pot model with heavily radius pickups) - same as above … fast, light as a feather but no neck dive, excellent flexibility in tone. I can pull everything I need from it for country, top 40 (more slapping), classic rock, or backing acoustic gigs (two singers and an acoustic guitar. I love it. But … the E and G strings are weak no matter how the pickups are set. I’ve read a few complaints about this, and Ibanez has redesigned the pups and I’ll be replacing the stock ones for the new style soon. This bass does every I need, it covers every style I play when doing fill-in work and covers my full-time New Country gig. I pretty much set my amp flat and use the volume, treble and bass pots to adjust for the tone on a specific song or adjust for a room. I’m always impressed with how flexible a $399 bass can be. My only issues have been the pup radius, and that the frets were not dressed well past the neck joint.
SR400 w/ rosewood (?) board and new style pots and pups - feels and sounds essentially like the SR300/370. Again, the strings have an equal volume everywhere due to the redesigned pups, but it hangs, plays, feels, and sounds just like the 300 series. Love it. Not sure what the difference is, the weight feels the same, neck feels the same, I probably couldn’t tell if someone was handing me a 400 or a 300 if I were blind folded.
SR500 w/ rosewood (?) board - noticeably heavier than the 300 series, so also no neck dive, of course. Neck feels the same as the 300s to me. Same tone flexibility except the Bart seems to need a little more treble than the CAP pups for sets that need a clear very hi-fi sound. String volume across the entire fretboard was even. No hotspots or dead strings, no weak strings. Excellent tone, and sustain. To me it is everything the 300 and 370 is except more even string volume and slightly heavier. I’d probably love an SR500M.
SR700 w/ rosewood (?) board - everything said about the 500 goes here, too. The neck feels different from the 500/300 somehow. Not sure how or why, cuz I think it’s the same. It may be the strings were very different? Possibly feels slightly heavier than the 500, but still not as heavy as the SRX 700. The SRX700 feels like a Les Paul when strapping it on after playing the 370 for an hour. The SRX7 can actually give me a bit of shoulder ache over a 4 hour gig or long practice. The SR700 is not that heavy. Just not used to the weight since my six strings are all G&L Legacys or Gretsch 51XXs. The SR700 just feels more 'substantial' somehow. It might be my eyes telling me things I don't really feel, but it feels more expensive than the others. It feels high end, even though it's not much more than the 500 and is probably similar in pups, preamp, woods, etc. No fret sprout and they were fully dressed all the way down.
YMMV of course.
