^I also own the Klipsch Promedia's and yeah, they're great too although the Logitech's are a bit cheaper and play a little louder
Yeah, Logitech says "400 watts" on the Z623 box but of course that's peak power output which is silly because who's going to complain about 200 watts RMS, particularly from nearfield speakers? They're loud enough that i've actually made two different houses i've lived in with them, rattle. You're also correct about there being no sensitivity rating for them but then i've never seen any sensitivity rating for any 2.1 systems and I own a few. Few subwoofers include that info either. For some reason we're only privy to that information on normal 2 speaker setups but yes, speaker efficiency is not a trivial spec. As for their frequency response of 35hz-20khz, again you're correct, there's no way they're doing that at 200 watts, more like 5 or 10 watts because the higher the volume the worse the frequency response.
I was just checking out the stats on your LSR305's (which i've heard a few audiophiles recommend) and their frequency response is 43 Hz-24 kHz which is impressive for an inexpensive pair of 5" studio monitors, especially from JBL so it's no wonder so many people like them. I also own two pairs of 5" active/powered speakers (similar to the JBL's) by Kanto and Audioengine but neither dig as deep as your LSR305's and my speakers were more expensive. The weird thing about the LSR305's is that JBL claims they produce 41 watts to each tweeter and 41 watts to each woofer. Firstly, why be so specific and say "41 watts"? Why not just say 40? More importantly though, i've never heard of any active/powered speaker system (and they're kind of my thing) whose amp produced the same power for the tweeters as they do for the woofers since bass requires much more power. For example the Logitech's claim 130 watts to the sub and 70 watts to the two satellites and my other systems are similar, sending about double the wattage to the woofers/subs than the tweeters