AV receiver a better bet than my current setup?

It looks like you have stellar gear. Unless you are dealing with budget issues,leave the home theater receiver for home theater. 

Is this a joke?

Nope - no joke - it actually happened. Thx for the compliment on my gear; and although I wouldn’t call him an idiot, I hear where you are coming from. :)

I wonder if there was just a misunderstanding between you and the store owner.

Definitely not. but thank you.

I wonder if there was just a misunderstanding between you and the store owner. If not, go to a different store. I've never heard an HT receiver, at any price, equal a good 2-channel stereo system for playback of 2-channel stereo music.

The  Backert Labs Rhumba pre-amp has HT bypass functionality. You can "add" HT functionality to your system, still using the Krell and B&W speakers for the front L&R speakers/amp without affecting the sound of your 2-channel system at all. You could simply buy a decent HT Receiver that has L&R pre-outs, connect the L&R pre-outs from the HT receiver to the HT bypass inputs of the Backert. Then connect center channel, surround speakers and subwoofer to the HT receiver for power. Finally, connect any HT source (DVD/BlueRay Player, TV Cable Box, etc.) to the HT receiver. 

I've set up "combo systems" like this on multiple occasions. Works like a champ. When playing 2-channel music, you can turn off the HT receiver. No affect at all on your 2-channel music system. When playing HT, turn on the HT receiver, turn off the Backert and now the L&R HT front channel content is directed through the Backert to the Krell amp and B&W speakers. 

You can add HT without affecting the 2-channel stereo music playback system you already enjoy.

Is this a joke?  You’ve got some GREAT electronics and am surprised you’d even be taken in by the salesman’s BS that you should consider ANY receiver over them.  If this is true, NEVER speak to that salesperson again as he/she obviously has NO idea what they’re doing and just trying to make a sale.  If I’m not mistaken, Backert may be one of the few tube preamps that can deactivate their tubes while being in HT bypass mode, and if this is the case it would even make it an easier decision to get an AVR with front pre-outs so you can just run them into the Rhumba and have the best of both worlds with the push of a button.  Either way, RUN AWAY from the ABSOLUTE IDIOT who’s recommending a Marantz AVR over your excellent electronics.  Yeesh. 

Do what I did. Find the best HT you can, bring it home, compare side by side with whatever leftover solid state amp you can find from the 70's.* Be careful, possible side effects range from laughing yourself silly (that anyone buys into multichannel) to spine/nerve damage from shaking your head in disbelief, to falling into a Heisenberg fugue state at the psychic disconnect over what HT promises vs what it actually delivers.  

Then call the dealer ask him to come pick it up. Explain he might want $1500 but it is not worth the cost of the gas to drive it back, so come and get it.   

*(Not a joke, not hyperbole, I did this with a 70's era Kenwood KA8004 integrated amp.)

Thank you.
I doubt it as well.
Cheers.

I highly doubt that a Marantz or any AVR will sound better than your Krell and Backert setup. AV receivers just don't sound very good, I have sold all my surround sound amps and preamps after trying for 20 years. AVR's don't come close to the equipment I got rid of. Maybe a Trinnov or STR preamp along with separate amplifiers would be okay, just not for me.