What front end piece should I upgrade?

It appears you try "low end equalization" with added distortion. Why? Are you lacking bass?

It sounds like you could eliminate the use of the EQ and add a subwoofer to fill in the richness of low end. It appears to sound just fine otherwise.

Does the Jr. have a built in preamp? If so, have you tried playing with volume level on it? When I had the DSD MkI best sound was with volume set at 92. Reduced the overall noise. One other suggestion, if you’re using RCA cables, try XLR. You might get your bass improved a bit and it might sound better overall. Also, Kimber cables are not warm and paired with open and resolving Sim Audio amp could be the source of your fatigue. You can try different speaker cables. 
This is before you upgrade anything.
Now to the upgrade…depending on your budget, there are better DACs out there. I’m using Chord Hugo TT2 now and it has a good amount of tuning as far as output is concerned. It has a super low noise floor, is more resolving than the PS Audio DSD MkI but not as warm sounding. It could be the upgrade you’re looking for. Then there’s always a DSD MkI that’s going to be an improvement over your Jr. and can be had for under $1500. It’s going to retain the PS Audio sound signature but should have better bass and should be better overall.

There are many other good choices out there. What’s the budget?

Well, there are certainly better DACs out there that would likely be less fatiguing.  What’s your budget?  I’d also upgrade those old Monster interconnects as they’re not likely doing you any favors either. 

Budget would be helpful.

Does the PSAudio DAC have options for different digital filters?  If so and you haven’t experimented with them, I would at least try that.  Also make sure that you have the latest DAC update; you may need to contact PSA for that, as at least at one time they were doing them via flash drive and not over the Internet.  
  I have read of a lot of users being unhappy with that DAC, fwiw.

  If all that fails I would get another DAC and will defer any recommendation until we know the budget.  You may also consider a streamer/DAC combined.  I don’t know much about the source computer you are using.

  The rest of the system seems ok.  I would ditch the Monster cables and that should help but I don’t think that will solve the issue .

WOW, thank you everyone for getting back so quickly with great ideas.

First, I didn't include a budget, not because money is no option but because I am interested in better matching my components, particularly matching the front end to the amp and speakers.  Since I have $8000 invested in them and $2000 in the renderer and streamer (which so far no one has indicated to be a problem), I am not sure how much to spend on a DAC that would be comparable to my system.  I certainly wouldn't want a high end DAC that would be limited by my other components.  

Regarding the lack of bass, I did have better bass with the Oppo DAC, but much more noticeable harshness, smaller soundstage and less definition.  I have read that the PS Audio (even the MK1) tends to be rather weak in low end reproduction (I am using the last software upgrade and have the volume set at 92).  If I could get rid of the harshness that causes fatigue (and it appears my cables may be related to that), I could possibly live with the Roon equalization.  In the Roon discussion forum, several individuals have commented that employing any of the DSP features tends to create a bit of noise similar to what I have found, and in my system this noice seems to add slightly further to the harshness that I wish to reduce.

However, I appreciate the comments made about both the balanced Monster interconnects and speaker cables (I am currently bi-wiring the old blue & white Kimber 4TC). So I will definitely look at upgrading them.  I would appreciate suggestions for replacements, again focusing more on achieving a better match for my system.

Hope I have addressed all the questions and comments and appreciate the education and responses.  Thanks

Given what you’re looking for and the level of your system I’d highly recommend doing a trial of the new Linear Tube Audio Aero DAC as it will address your harshness issue and likely provide significant improvements across the board. I think it could be revelatory in your system, but if not you can just return it.

https://www.lineartubeaudio.com/products/aero-dac-digital-analog-converter

For speaker cables these AZ Satoris are excellent, and if they don’t provide a meaningful improvement you could easily sell them for little/no loss or even a small profit, so why not?

https://www.usaudiomart.com/details/650129691-acoustic-zen-satori-shotgun-bi-wire-8-spade-pair/

For interconnects you can do a free trial of these, and as with the above speaker cables I think they could provide significant improvements over your old Monsters for a relatively very low price.

https://iceageaudio.com/ice-age-audio-store/ols/products/oxygen-free-copper-xlr-interconnects/v/OFC-XLR-3-FT

Hope this helps, and best of luck.

You experience with that DAC is not that unusual.  Wonderful unit but soft bass is a critique I have heard from more than one customer.  I would look at new or used Bricasti M3.  It is a very common upgrade for folks moving from PS Audio DACs.  You could also look at the Rockna Wavellight or Canor DAC 2.10.  These are all going to be neutral to a hair warm and will have great bass response. 

The other brands that may do well for you is Aqua.  I don't know the models all that well but I know they will have a sound profile that should meet your needs. 

Full disclosure, I am a dealer for Bricasti & Rockna, I am the importer for CANOR and have no relationship with Aqua

Get the Sonore OpticalRendu to replace the ultrRendu. Fibre Optic cable is something I can easily hear the benefits. Fibre is made of glass and glass cannot carry analog noise from your network into the DAC. Your ultraRendu likely does a good job, the opticalRendu would be better.

I have 2 systems with 2 network switches that provide the gateway to a fibre optical signal. One cost $79 and the other was $150, though the $150 was around $500 on Amazon recently. 

BTW - I have the Sonore 'audiophile' power supply and other audiophile stuff for my 2 streamers. If I were to do this again, I would save my money and use the regular power supply and cheaper whatever. As long as you have fibre just before your DAC then I do not think you will hear the difference with gear before the fibre.

@seaside 

Your system looks great, but if I was to nitpick a little, I’d replace those Monster IC’s with some a bit more up to date, like something in the midrange of the Audioquest line. ( look for used!)

If you are looking for more bass, look at a SVS PB 1000 Pro or 2?

All the best.

Keep playing with the Roon dsp. Try a parametric eq boost or cut centered at 5khz and from from 4-6khz to change “pierce” effect in the sound to your preference. I like a 2-3 db boost there on my systems with both Sonus Faber Concerto Domus and Ohm Walsh. If your room is acoustically challenged like mine a lot of DSP room correction may be needed for best results regardless of what gear you use. Also a steep high pass filter set at 20-30 hz or so would place less demand on the amp and improve performance. You have good stuff assuming all is in good operating order. If not then get repaired or replace. Tweeters and crossover in good shape? Roon DSP should be able to take things to a much better place if needed including applying a custom room correction filter. You can create one of those for your room using REW freeware on a laptop using a decent quality microphone. Good luck!

Thank you everyone for the input.  First, I will make the suggested changes to the Roon parametric equalizer.  Second and after reading reviews on the Kimble 4TC, I appreciate the recommendation to upgrade them.  Most reviews indicate they are best for satellites with a subwoofer (I used to have KEF satellites with a subwoofer but they did not work in the larger room in our new home) as the Kimbers were found to have weak bass and the high end was somewhat bright and harsh.  Those AZ Satori Shotgun (used) sound great.

Third and moving onto the recommendation to upgrade the Monster XLR interconnects, I should have said that they are M1500s with a retail price in 2005 of $400, although I purchased them used for $125 when I got the PS jr.  Thus, I never listened to the DAC with any other interconnects.  According to reviews(3 combined into one), "The M1500s are in a different league from their much cheaper brethren. Their build quality is substantially superior (they have very sturdy yet flexible outer jackets) to the M850i, M950i, and M1000i cables.  In fact, one of its strengths is the connector. It uses a solder less design that was employed on the Sigma. They were also much more expensive new.  I would say that they are a fairly good and neutral cable, and that they compare rather favorably with the Audioquest Emerald's that now reside in my system."  

So with that new info, do folks continue to believe that I should upgrade them?

After I complete those steps, I will weigh whether it is better to upgrade the DAC or move from the ultraRendu to the opticalRendu.  These would clearly be more costly upgrades and if the changes to the speaker cables and interconnects would resolve the harshness and begin to address the weak bass response, I may be able to live with my ultraRendu and DAC with some tweaking of the Roon parametric equalizer.

Please let me know what you think and again, thank you all for your suggestions and expertise!