It sounds like one of the small air tubes located inside the the bag cover may be blocked. I recently had a similar issue with my Model G. After stripping the paper sleeve of a twist tie (with a needle nose pliers) I used the wire to clean out the metal nipple on the hose inlet to which one of the tubes attached and voila, it now works just fine. To access the tubes just remove the screws of the inside cover plate and remove the plate (at least that's how the Model G's tubes were accessed). Before reattaching the cover plate completely, check to see if you have corrected the problem. If not, check the other tubes for blockages. By the way, I tried using a bent paper clip, too, but it was too large in diameter.
As for retrofitting your cleaner with an adjustment dial ... that used to be done back in the day by the friendly Electrolux men (shortly after the Model LX made its debut in 1952). However, they would have had the correct tools and parts. "Tig21er" (John) is a 50+ year Electrolux sales veteran who continues to sell Aerus products. If he (or another person who has performed the task) doesn't respond to this thread you might try emailing him for information. It is doubtful that the average Aerus representative would know how to attach a dial, but if you have an Aerus store in your area you could always check with them.
Charles, if you have an actual close-up photo of a basement and attic key I'd appreciate seeing it.