Home Theater Receiver recommendations for B&W 803S?

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. 

Oh, you already have the Panasonic DMP-DB35.  That doesn't even have a digital COAX, so I would not bother getting coax cables now, unless you want to.  It's not worth it to try a good toslink cable because you are already thinking of replacing the Panasonic. lol.
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Those Silverstar fuses are pretty much a steal at this price, and because they are 100% silver, they are definitely what you need to add speed to Marantz.  Normally, the upgrade fuses are in the $60-90 price range and are definitely expensive.
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The Power cord $50 estimate is actually extremely low.  If you were able to DIY a bit and cut/reterminate a plug, then I could probably recommend something anywhere from $150 to $450 investment for a really good power cord.  Otherwise, for $50 range, you are looking at something like this:
https://www.ebay.com/itm/1-5M-5ft-Element-Power-Cord-Cable-Wattgate-and-Marinco-HighEnd-Audiophile-U-S-A/253938496557?hash=item3b1fe9f42d:g:GYUAAOSwHPNbKEpK:rk:7:pf:0 
It won't be a great power cord, but the Wattgate connectors will increase speed and impact.  The problem is I have found that the brass connectors can "push too hard" and become harsh sounding. 


Here's another option if you want:
https://www.ebay.com/itm/5-Ft-Silver-Plated-12AWG-IEC-AudioPhile-Power-Cord-Cable-W-Wattgate-Connectors/151904760005?epid=859384614&hash=item235e3ab0c5:g:4PkAAOSwv-NWYRZg:rk:8:pf:0 
silver plated wire with wattgate connectors - will push super hard on the voltage.  

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In my opinion, the best way is to get a very nice solid core power cord and then chop the ends and re-terminate using a Furutech silver plated IEC and a Cardas rhodium/silver male plug.  This could be a $200-250 investment or more.

Thanks for all this... will definitely have some research to do this weekend!

On the digital coax (which I do intend on doing once I replace my disc player), you said to make sure it's minimum 6 feet. I'm curious, what difference does that make?

This only relates to digital cables.  If the cable is too short, what happens is when the digital pulse hits the target device, some of the charge will actually be reflected back to the source equipment.  This "reflected pulse" will interfere with the newly generated pulses and actually cause some of the pulses to be cancelled out, therefore creating errors in the digital transmission.  The result can be heard as smeared or lower resolution sound.  So, for any digital cable (digital COAX, USB, HDMI), I always recommend at least a 6 foot.  The experts say that you should have a minimum 1.5 meter cable (4.5 feet), but I always go 6 feet to be safe.
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This does not apply to power cables or interconnects.  Power cables and interconnects can be as short as you need them (but I always say it's good to have just a little bit extra length in case you need to move equipment around a little).