OB1 versus IB1?
The mid-range driver and tweeter are the same for these speakers, but there is a huge difference in performance between them, because of the much bigger bass driver of the IB1 and IB2.
If you desire extended bass with no problems with bass power handling, then buy IB1s (or IB2s).
If however you play your music at moderate levels, and never crank up the volume, then buy OB1s.
I have owned OB1s and currently have IB1s, because they go very loud and very deep, so much so I will never need a sub-woofer.
The OB1 doesn't have the same efortless bass and volume capacity as the IB1/2, and I found that a TLE1 sub was necessary.
I found it was all too easy to overload the bass drivers on my OB1s when trying to reproduce deep bass at realistic levels ( I like my music LOUD!)
A single 4B-SST has more than enough power to drive my IB1s to very loud levels, as it delivers 500W per channel into the 4 ohm load of the IB1/2. My room is 18' x 12' and I have no problems with the speakers exciting room resonances, which I believe is a major plus point of transmission line designs. My speakers fire down the length of the room.
It is worth bearing in mind that the OB1 is a 'domestic' speaker, whereas the IB1 is a true studio monitor, and as such is designed to take some 'abuse' without complaining.
If you can afford them, buy IB1s or IB2s, because I doubt you'll ever feel the need to replace them.
For more info do a search for my other postings on this forum.
Cheers!