Are you running the 980H via HDMI? If so, I doubt there are any useful tweaks. If not, you should.
The 980 does benefit from a better power cord, naturally. How better? Depends on the cord being used.
Different footers too will help. Also placing some weight (not a lot) on top will help out too.
of course so will a good stand or rack to put it on.
An outboard DAC is (depending on the DAC) gonna really change things... and for the better too.
I use mine primarily for DVD via HDMI, and 2 ch red book, via a Stereovox XV2 digital link into a BC DAC3.
As to modding it.... Why?
I believe thoughts on modding a player IMO might be better suited to another step or so up the line, or one with a better transport.
With a DAC which de-jitterizes the signal well, it'll come down to how much denairo ya got, and how picky are your ears. Running both a SCD xa 777es, and the 980 into the DAC 3, the sonic diffs were quite minimal.
If a one box deal, and multi ch are your preff, go for it. I would consider the 983 as well, though.
I've had two modded Oppo players, a 970 and a 981. The simplest area to do mods on those is on the power supply board, since the power supply board could be easily removed and the components are fairly easy to access. And, no surface mount technology. I hope that's the case with your 980.
The mods on my players were more extensive, in addition to the power supply upgrades. The 981 had several capacitors and the stereo-channels op amps upgraded. (Not sure of all the details, since I bought the unit already modded.) The 970 had almost every capacitor (a lot!) upgraded but no op amp upgrades. A friend of mine upgraded the 970. He said that the power supply upgrade was no big deal; but as far as upgrades on the main circuit board (which takes up most of the chassis)--well, I doubt that he'd be willing to do that again.
So yes, audio performance (specifically, from the analog outputs) did improve, but it never really impressed me as "audiophile." I doubt that it's on par with your modded Sony.
I sold the 981 (nice player, I just wasn't using it) and still have the 970, which I use for watching movies and for multi-channel listening. FWIW, I don't use a processor. I feed the analog outputs right to a multi-channel amp and a powered subwoofer, so I have sort of a minimalist HT system. I'm actually quite pleased with it. I just don't think of it as audiophile.
Back to my earlier point, a power supply upgrade might be the most cost effective upgrade, and may well improve both audio and video, including audio through the HDMI. And it's relatively simple to do. As for the rest, well if I were going for audiophile-caliber multichannel sound, I'd be looking at a different player.
I am running a 981 to my 8.8
I have done the following - never did A/B before and after, just sort of did it by reflex
I used the Verastarr damping material liberally inside the case. Its one of those sheet metal jobs that goes thonk when its stock. Now its more like thud
I use an Oyaide C7
Because space is extremely limited I use a set of Herbies Tenderfeet underneath it - I would top load if I had room as well
I run a Wireworld HDMI out
As I said, no A/B but it works really well
Waiting on the new Oppo BluRay - assuming that it will also play DVD well, I will scrap the 981 and the Pannie BD30 for the one unit. At that point I may also consider a modder upgrade package
I just found a fairly inexpensive tweak for the DTC 9.8 that seem to provide a decent improvement in audio quality. I saw that MusicDirect had Marigo VTS dot sets on sale for half price. I bought the one for a DAC. It comes with 10 3mm green dots for capacitors, 10 2mm white dots for solid state components (ICs, voltage regulators, DACs, etc). 4 6mm black dots for the chassis front and rear and 4 2mm green dots for chassis top and botton.
I put the 3mm green dots on top of all the large capacitors, I put the 2mm white dots on the voltage regulators (ones with heat sinks), HDMI chips, Toshiba DAC.
Installing the dots is a little tedious as they are so small, but using an exacto knife helped.
After completing the install, my first test was a MCH SACD of Diana Krall - Girl in the Other Room. I found quite a few areas of improvement. The lower bass had more definition and weight. More clarity in the midrange and transients seemed less smeared. Overall it seemed more like I was listening to real people playing real instruments...I then popped on several Telarc MC-SACD which confirmed my initial impressions. Then I switch over to my AppleTV streaming Apple lossless files connected to the DTC 9.8 via toslink. In the past I have found this source to be less than wonderful. OK at low volumes...but strident as I would increase the volume. Now it is quite enjoyable...and more musical.
I was hoping to move the DTC 9.8/Oppo combo closer to my my VSE Mod 4 Sony DVP-9000ES through my 2 channel Air Tight ATC-2 tube preamp...In stereo mode it was not very close before...They have definitely moved closer...too soon to tell yet exactly how they stack up.
A pretty impressive improvement for $50 and a couple of hours...I have used Marigo VTS dots previously, years ago in my 2 channel system on DACs and preamps with pretty good success. Seems like it applies to the DTC 9.8 as well.