Chris, I have owned two pairs of SBLs (mark 1 and 2) passively and extensively heard the Iota Explorers at the end of a 500 system fairly recently over a few separate days.
The Neats are very musical speakers. Highs were a little shouty when volume was pumped on those ribbon tweeters.
IMHO, they are still not at the SQ level of a pair of fully optimised mark 2 SBLs, even now. Maybe you could source the mid-bass upgrade to the mark 2s if you can find them, and get a 52 which will really open up the SBL’s scanspeak tweeters much more. Have you changed the tweeters because after 30 years, they should be refreshed. The mark 2 SBLs really sing beautifully balanced at the end of a 52/135/CDS2 set up, and if you, I would work towards this combo before looking at a speaker change.
Active set-up = more outlay and boxes. The Archilles’ heel is not in the speakers assuming their drivers have been refreshed.
I would use a minimum 52-250 for the SBLs, but the immediacy and grip of the 135s is something else.
I have recently had feedback from a reliable source that the 250DR is really no slouch and a different kettle of fish to the older olive 250 you have, so it might be a consideration for you with less boxes against the 135s, though the aesthetics would be a mix and match olive-black box, if it doesn’t bother you. I prefer all olive, or all black, but that’s me.
The Iota Explorers are close, as a “mid level” Neat speaker, but not quite as refined in direct comparisons to the mark 2 SBLs. They have an SBL-like bass (80% for me), good mids and PRT, but this is at the end of a full 500 set up with the ND555. Anything lesser you’d have to try a loan. Somehow, they weren’t enough for me in overall terms compared to the still-mighty mark 2 SBLs.
Please cable dress, and look at your support rack and mains if you haven’t, because these when combined, help your olive set up without you having to spend on components. Cheers