What front end piece should I upgrade?

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.