>I think the majority of Spectron owners end up getting Joule-Electra preamps
That's definitely not true, but it is a popular pairing, partly because there's a business relationship between the two companies (via Simon, at any rate).
I tried an LA-150 with my Musician III for a couple of days just two weeks ago, though my speakers aren't Maggies. The LA-150 is a seductive preamp, no doubt about it. Rich, ripe, gorgeous tone color; a sweet and delicate treble, though not extended; full (perhaps overly so) bass, though a little loose. Ultimately, though, it was less detailed and resolving than I like, with compromised image specificity. It's sonic signature is quite strong and was evident on all recordings I played. And while I sat in awe of the gorgeous tone, I found myself bored by the music with the Joule in my system. I'm sure others will disagree vehemently, so take my comments with a grain of salt. I wish I could have kept it longer because I didn't have time to play with cables or other tweaks that might have made a difference. Lousy remote.