Check if there is dust on the lens. If clean, try unplugging the unit for about an hour and pushing all the buttons (sounds like b.s. I know but it cured by CDP of issues). Good Luck!
We need to know w lot more about your setup before a credible response can be crafted....
Main/other speakers?
Preamp/amp/other?
I personally set the mains to small & try shifting the crossover frequency up/down until I get a good blend. You can also play with the slope...
I had one of the Tube Preamps but sold it to fund the TG-IV & 400x7 amp. They are about the sweetest sounding preamp I've ever heard....kinda wish I'd kept mine.
What amp do you use with yours?
A PERSONAL pair of Bob Carver's tube monoblocks made out of a collaboration between Bob & Tim de Paravicini. 185 wpc personally handmade by Bob. Shweeeet!
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http://www.carveraudio.com/phpBB3/viewtopic.php?f=113&t=5340&p=51703&hilit=bob+carver+tube+amp%2A#p51703
You shouldn't need a Y splitter; most H/T receivers, as well as the subs themselves, have provisions for multiple subs..either thru daisy-chaining or even mulitiple outputs.
I have a 7.1 with the RT-12d sub and I've had a LOT of audiophile friends (Carver/Sunfire enthusiasts) comment on how well it blends. The digital correction feature is SHHHWWWWEEEEEET! I've never looked back.
Stupid question in response to your original question; are you SURE all the speaker wiring is in phase? If you've accidently inverted one it will sound like what comes out the south end of a northbound moose....[]
I'm using a pair of Carver Amazing Loudspeakers-III (the tall, narrow ones) for my 2 channel system; they set a FANTASTIC soundstage. Driving them with a Sunfire 400x7 amp, bi-amping the ribbons/woofers with separate channels from the amp.
Rear surrounds in that configuration wouldn't have a lot of info going to them. I'm using KG 1.1 bookshelf speakers for my rear surrounds with no issues. IMHO RB-75s would be overkill.