Home Theater Receiver recommendations for B&W 803S?

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).

@awilder

In regards to your Marantz 8802, I bet the Audyssey was still turned on from a previous owner's configuration and I suggest you turn the Audyssey calibration off. That was probably the reason why the performance was a bit off. Until you receive the Audyssey setup microphone then you can start doing your own calibration for your room, or you can also leave the Audyssey off the whole time. It’s up to you to decide whether you like it with Audyssey on or off.

You should upgrade the power cords for your Marantz 8802 and your Rotel RMB-1585 as well as for your bluray player and other source components. Especially the Rotel RMB-1585 is a high current five-channel power amp, you definitely have to upgrade the power cord for it.

The Marantz 8802 was great for home theater use and performs almost flawlessly. I have heard it in my own HT setup and system before paired with all my Classe Delta series class AB amps on the B&W 800 D3 speaker surround system. My cousin also owns one paired with the same Rotel RMB-1585 amp and the combo performed really well on his B&W 804 D3 surround speaker system.

However, to me the 8802 isn’t good enough when used as a stereo preamp for stereo music playbacks. Maybe I have high expectations cause I don’t use my AV preamp processor for stereo music listening since I have a separate reference setup and system in a separate dedicated well treated 2ch listening room that consist of cost-no-object design gears and accessories and high end cables, etc.
If and when budget permits you should consider getting a good quality dedicated analog stereo linestage preamp that has HT bypass input so you can connect your Marantz 8802 pre-out for front LR channels to this HT bypass input on the stereo preamp thus disabling the volume in the stereo preamp when this HT bypass input is selected. Keep the Marantz 8802 for surround duties only. Then for your stereo music playbacks you will have to get a good quality digital front end components (CD/SACD player or/and a streamer/DAC or a standalone DAC) and connect via analog to a stereo analog linestage preamp. So when listening to stereo music the Marantz 8802 isn’t in the signal path at all and you don’t even have to turn it on. Your bluray player and other AV sources are still connected to the Marantz 8802. Basically just leave the Marantz 8802 for surround duties for movies (HT).

For your stereo music source component if you can find a used Modwright Oppo 205 player and you can use this for everything : CD/HDCD, SACD, DVD-Audio, bluray, DVD Video, 4k UHD bluray disc playbacks and run the 205 via HDMI & coax to the Marantz 8802 for surround movie playbacks and run a pair of stereo XLR balanced interconnects out of the Oppo 205 directly to a stereo analog preamp for 2ch music playbacks. The Modwright mod only affects the stereo analog audio output stages on the Oppo 205 and this is the best mods that Dan Modwright has ever done. Comes with an external PSU unit that feeds power to the stereo analog audio output stages only. The Modwright mods are tube based. The Modwright Oppo 205 includes total redesign of its stereo analog audio output stages which includes replacement of op-amps with a discrete class A output stages in its stereo analog output stages as well as true differential or fully balanced design implementation with its stereo analog output stages. This mod also includes further dampening of the Oppo’s disc transport. This fully modded Modwright Oppo 205 will elevate the player’s performance to near reference quality.
I have heard this Modwright Oppo 205 before in my own reference 2ch setup and it was very good. It’s highly musical and very analog sounding while it doesn’t perform on the same level as my DCS Vivaldi full four stacks digital front end components but I must say that the Modwright Oppo 205 is still quite impressive considering the added total costs of the modifications and a cost of the stock Oppo 205 alone.

But all these will cost you quote a bit : good quality stereo analog linestage preamp and a good quality digital front end source component(s) for your stereo music listening needs such as the Modwright Oppo 205.