Need recommendations for better two-channel audio out of my Marantz SR-6005 and my B&W 683

#1 First things first,
Get rid of the lousy Klipsch sub and get two Rythmik sealed servo-controlled L12 subs The price is 1060 usd shipped (530 each). This alone is a great future proofed upgrade for music. No matter where your upgrade journey takes you, these subs will continue to stay. Keep in mind, the subwoofer is essentially a speaker and it has significant impact for music next to your 2 main speakers, IF you have the right ones.
http://www.rythmikaudio.com/L12.html

#2)
If you insist on keeping the receiver in your 2 channel system, get a Marantz PM8006 integrated amplifier. It has a power amp direct in (bypasses the preamp) that you can connect to the receiver pre-outs. See how it sounds. You could also get the pm8006’s preamp in the chain, run your calibration and see how it sounds. The advantage here is you get to keep the bass management from your receiver.
Here’s a factory refurb with a 3 yr warranty for 850
https://www.accessories4less.com/make-a-store/item/marpm8006/marantz-pm8006-2-ch-x-70-watts-integrated-amplifier/1.html

#3)
If you’re still unsatisfied, lose the receiver altogether for music listening, get a Marantz NA6006 streamer and connect it to your PM8006. This is a great sounding streamer+network player+DAC+Preamp at this price.
Here you go, factory refurb with warranty for 500 bucks
https://www.accessories4less.com/make-a-store/item/marpm6006/marantz-pm6006-2-ch-x-60-watts-integrated-amp-w/d-to-a/1.html

The NA6006+PM8006+your speakers+ 2 Rythmik L12s will get you golden once you take the time to integrate your subs properly (it is difficult without bass management, but, you will get there)

Thanks for the replies @soix and @deep_333, really appreciate it.

1) I see a lot of dual sub posts on reddit r/audiophile. I will definitely plan on upgrading the sub, could always get a second down the road.

2) I already have a pretty nice two-channel system in another room (Jolida fusion dac pre, Parasound A23 amp, Meadowlark Kestral speakers). I use my computer as a source and run USB to the Jolida, it sounds great. What I'm trying to do here is improve the two-channel audio on my home theater system. I have to keep a receiver in the loop since I also have B&W center and surround connected, along with a 65" TV.

I think I will try hooking up my Parasound amp to the pre-outs from the Marantz and see if it improves the sound at all. I could always leave the Parasound in the home theater system and get something like a tube amp to replace it in my two-channel.

I'm confused when you say "You could also get the pm8006’s preamp in the chain, run your calibration and see how it sounds. The advantage here is you get to keep the bass management from your receiver." If I'm using the pm8006 pre-amp stage, how is the receiver in the loop? Would it be digital source -> Marantz -> pre/outs -> pm8006 (not in direct amp mode)? I guess this would be the same as digital source -> DAC -> pm 8006 (or Yamaha AS1200).

Thanks again for the suggestions. You've got me reading about subwoofer phase alignment on the Rythmik Audio website!

@georgepv , yes, the Rythmiks have way more DOFs (full 0 to 180 variable phase control, PEQ, etc) to help integrate a sub better than many other subs out there for 2 channel systems. Sealed/servo-controlled is the way to keep it precise and clean for music and they do dig very low at 12 to 14ish hz. It is a steal at this price point.

PM8006 has a poweramp direct in, which bypasses the integrated amp’s preamp section. It would be the exact equivalent of using your A23 power amp on its own with the receiver’s preouts. But, you also have the option using its preamp (a.k.a use it as an integrated amp in a dedicated 2 channel system). I was trying to suggest a "step by step" upgrade path in my earlier post.

But, since you mention that you already have a separate 2 channel system and want to use your 5 channels for multichannel as well (music and movies?). your best bet would be something like the Yamaha RX-A3080 receiver.
https://www.accessories4less.com/make-a-store/item/yamrxa3080bl/yamaha-rx-a3080-9.2-ch-x-150-watts-a/v-receiver/1.html
It is Yamaha’s current flagship (before they start rolling out the 8k receivers) and you can get a lower price on it these days. It towers over the Denon flagship and the Pioneer Elite flagship i have compared it against for MUSIC (It will walk all over your older gen Marantz receiver though i have not had a chance to compare it against Marantz’s current flagship). It is perhaps the best receiver and 1 box solution i can think of for 2 channel and multi-channel music without emptying your wallet completely. I have a significant collection of multi-channel SACDs and blu-ray audio i listen to with this receiver (in addition to 2 channel sources). It has been stunning for the past couple of years! Without stating the obvious, it would be a no brainer for your movie collection as well.
A.Robinson just recently reviewed it here.
https://www.youtube.com/watch?v=fBifGxSda7s

One thing to be wary of..... the RX-A3080 is jam-packed with enough features to overwhelm a multichannel noob. But it is well worth the upgrade once you study the manual, ask questions and figure out how to work it. In other words, it may overwhelm someone who’s buying a receiver for the first time. But, it shouldn’t be a problem for you, since you have dealt with receivers before.

Best,

Agree with @deep_333 that the Yammy Aventage AVRs, especially the upper models (1080 on up) that have a power transformer are very good. I worked for Magnolia for a spell and got to compare Denon, Marantz, and Arcam driving B&W 804d3s and thought the Yammy outperformed them all in 2-channel. Sound was detailed, transparent, refined, and imaged better than all others with a clearer background and a superior 3D soundstage. I was pretty surprised by this, especially regarding the Arcam. A customer who owned a Marantz AVR had me compare with an Aventage AVR, and let’s just say he was no longer happy he owned the Marantz.

That said, you can try subbing in your A23 and I’m sure you’ll hear an improvement, but you’re still stuck with a mediocre-at-best preamp section in the AVR that will continue to be a significant impediment to better 2-channel performance — hence why we were recommending using an integrated amp. If it’s me that’s what I’d do rather than switching AVRs. I’d think adding something like a NuPrime IDA-8 or the like would yield a huge improvement all around for not much $$$ (probably could use its DAC too). Anyway, hope this helps at least somewhat.

I ended up getting a Denon X3700H. Since this is mainly a home theater setup I wanted to stick with an AVR. After running through the Audyssey setup it sounds great. The speakers blend together much better in surround sound and the two-channel sound is significantly better as well.

This AVR also has two subwoofer outputs so I can add dual subs in the future once I refill my slush fund. This unit was a bit pricey but I am very happy with it. It gave me the level of detail I was looking for in two-channel and boosted my home theater as well.