I check the HB manual and they don't recommend the HI Cab/LO Cab combo unless you use 2-Mono 250's, because the HI Cab is 4 ohm and the LO Cab is 8 ohm. With only a Mono 500...they say your choices are Club Cab only, Club Cab/Low Cab (8 ohms each) or Big Cab by itself (4 ohms). I love the CC/LC combo...I've been trying to learn more about the sound of the Big Cab by itself...Schlobodan and Flo say the CC/LC and BC only are very similar though. IMO when you crank up the Mono 500 with the CC/LC...you get a full range sound, getting warmer and louder...not biasing any particular frequency. Hard to explain, but if you A/B it...you'll know!
I've bought all my HB equipment through Randy @Pulse Music (pulseonline.com) in Indiana, except my 5RU sleeve. Great guy to deal with...all my HB stuff drop shipped direct from the US distributor to my door...CC/LC cabs from Hanser Music years ago, and HB Pre/Mono 500 from US Music more recently. I have to say both distributers shipped very fast.
I purchased the 5RU sleeve from the NYC Custom Shop...great experience too. Tyler shipped that thing within an hour of purchase. Great guy to deal with also!
+1 on babebambi's comment about the access to the rear panel in the HB sleeves. I bought the 5RU so I could put a Furman PLC+ power conditioner, which has a 12v light switch/jack at the rear for an LED adjustable light...man, it's dark and tight in there!