I owned the 4.5's/RS-1b's/and then the Betas.
My friend still has a mega modded pair of 1b's.The crossover mod is a MUST!!Rear cabinet damping does not hurt either.
I did have a friend who had the II B's.I loved that speaker,and it is "well" worth taking it to the max,if you can.A superb speaker!
I felt the 4.5 was outpointed by the 1B's/and Betas.The II B's are more coherent too(than the 4.5),but a touch less powerful,since it has smaller(faster though) woofers.It really is a killer speaker,especially if in "as new" condition.A Classic!!
I remember my experiences quite well,as I think the Infinity speakers of that era hold up very well....Especially if you try to mod the crossover,and attend to some other tweakery.
BTW,there is a Guy named Bill Legaulle(spelling?) who lives in NY state(from the article I read about him awhile ago).He is an "expert" on anything Infinity,and has built up quite a following regarding re-voicing Infinity speakers,from that era.He actually rebuilds the Emim/Emit drivers,and sets them up as matched pairs for each tower.I know he has many Infinity followers,who have had him rework their speakers.He matches drivers to .5 db tolerances,per tower position. I have no doubt the guy is a "master" with Infinity/Emim/Emit stuff....Worth seeking out,if possible,but I have no more info about his whereabouts.
I DO believe I read about him in Positive Feedback,or one of the web mags.Also,one of my friend's "used" to know him well,and confirms his abilities.
Best of luck