Home Theater Receiver recommendations for B&W 803S?

I had originally recommended the Marantz because I am a firm believer in discrete analog stages.  It is pretty amazing that Marantz can deliver a processor that has 14 fully discrete analog output stages for $4k retail!!!  To get an 8 channel discrete processor, you have to look at the Bryston SP3 which retails for $9.8k or Krell S1200U which retailed $12.5k!!
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In all of my testing with different circuits and listening to many different systems, the devices that use op amps can sound good, but they just do not "sing" like a discrete circuit.  However, it could be that you might want to cut your losses with the Marantz and re-sell it.  In that case, I can think of a few options:

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Yamaha CX-A5100.  This will probably get you back closer to the sound of the Denon.  New is $2500, but used are in the $1800-2000 range.  There is actually a used one on audiogon for $1345.  Obviously, no discrete analog stages, but it may likely sound better than your Denon receiver.
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Anthem AVM 60.  This is probably the highest resolution processor in the budget range (before going to the Krell Foundation).  retail $3k.  Used in the $2200 to $2600 range.  There's a "new in box" one on ebay for $2488.
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Or go back to using your Denon receiver as a preamp for now.  You seemed very happy with the sound, but realize that there are some compromises to be had (i.e. there were some things with the Marantz that out-shined the Denon). 

Oh, no need to apologize!  I'm pretty sure we're going to be able to get he Marantz to blow away the Denon. I wouldn't be surprised if the Marantz is revealing other weak links in my setup.

I'm not opposed to tinkering with fuses and power cords..but let me first try the factory reset (and Audyssey configuration) and see how it fares after that.

Regarding power -- please try not to laugh: I have all my components plugged in to a verrrry old Monster Power HTS1000 (circa 1999...), running off a 6' extension cord (except for the Rotel, which is in a separate outlet). The outlet most convenient to my electronics is behind a bookcase, so I can't easily change out that extension cord; and I had to use an extension cord with a flat plug, to get the bookcase against the wall...sigh.

My understanding is that surge suppressors usually only have 10-year lifespan, so it's probably time to change out that power strip anyway?

Oh hey, maybe I can get a few bucks for my power strip!  Heh.

https://www.audiogon.com/listings/lis93f69-monster-power-hts1000-powercenter-free-ship-and-pp-ac-con...

Just saw your followup...

I’m definitely not ready to "cut my losses" on the Marantz! I think it is likely revealing other weak links in my setup, so want to address those first...many of which I’d also want to do, regardless of which preamp I have.

- Replace my Panasonic DMP-DB35 disc player (~$500)
- Play with speaker placement (free!)
- Try the Audyssey setup (free!)
- Switch from unbalanced to balanced interconnects between the Marantz and Denon ($150; may still be able to exchange the first BJC interconnects)
- Replace the Monster Cable power hub ($150?)
- Silver Fuse ($20)
- New power cord ($50?)

I would not worry too much about the age of the HTS1000.  The primary filter elements use film capacitors and coils, which really don't wear out.  The HTS1000 does use a couple of electrolytic capacitors, which do dry out over time, but unless you're getting a buzzing sound, you can definitely put a replacement power conditioner lower down on the priority list.  On that 6 foot extension, if it's not a heavy gauge cord, I would recommend going down to Home Depot or something and getting one of those extra heavy duty 12awg power cords.  They are cheap and the large 12awg will not restrict current.

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One other thing I noticed is you said you are playing CD audio over HDMI.  This is a bad bad thing (most people don't realize this).  I would highly recommend getting some digital COAX cables.  This is what I generally recommend for the money:

https://www.bluejeanscable.com/store/digital-audio/index.htm 

Get the Beldon 1694A cable and make sure it's minimum 6 feet (even if you don't need the length).  At $20.75 per cable, it's really cheap and does much better than HDMI.  Get one cable for each source device you have (bluray, cable/satellite box, etc.).  You should be able to configure the Marantz to use a different digital input for audio.  I would configure two different setups for Bluray.  One setup uses audio through HDMI - you would use this input configuration only for bluray discs that have TrueHD or DTS-MA HD audio.  Then create a second input configuration that uses that HDMI for video and digital COAX for audio.  Use this for all other things, such as CD and DVD movies.