Is my DAC my weakest link?

Concentrate on source and output (speakers). You got fine speakers. A bluesound node2i will give you good streaming and dac with subbass output if (when) you need more in the lows. Make it simple. Try to find out what sound you like. Move the speakers around in the room. Don't take your time and wallet comparing cables. Have fun!

I have a Marantz SA-10A CD player and DAC. It is as good as any source I have including Vinyl. CDs are superior to almost streaming service when played on top rate equipment. 

Since you are streaming Tidal Hi-FI through Audirvana I would recommend a DAC that can handle MQA. The Mytek Brooklyn is probably the best overall (for the price) but if you just want to get your feet wet you could try an Audioquest Dragonfly Red or Meridian Explorer2 DAC and either return them or keep one for travel or a smaller system (I have the Explorer2 hooked to a MacBook Air and powered speakers in a secondary office). 

If you like the sound but want to move up you could try the Project Pre Box S2 Digital or the new NAD C658. 

OK, so you bought an existing, older system. Most all of the components were of good quality, mid-priced at their time (except the DAC that was a budget item). Some of the components, such as the power amp, may be old enough that they need service (especially replacement of the capacitors). The real winner in your components is the speakers. If they are in good working order, the are worthy of upgrades to the electronics.

I agree with @towertone - A Pro-Ject Pre-Box S2 pre-amp/DAC could replace both your Rotel pre-amp and your HRT DAC. It has gotten really good reviews, it unfolds MQA from Tidal and it is really inexpensive at $399 MSRP. You can use it for your Tidal streaming setup and as a DAC to improve the SQ from your CD player. Buy it from a local dealer (if you have one) or a vendor that offers returns, such as Audio Advisor. Try it and keep it if you believe it provides an improvement in sound quality. If not, return it during the return period.

Past that, you amp (I'm assuming it is a B&K ST-140) might need service (capacitor replacement). You could also replace the amp with something like the  NuForce STA 200. There is a Demo unit on sale at Audio Advisor right now for $430.

Enjoy the journey. Enjoy the music. Don't get too caught up in the equipment, unless you have deep pockets. If you are enjoying what you have, don't buy anything unless you have the available funds. Spend time and effort on speaker placement and room acoustics. It's free (unless you buy room tuning devices) and can provide good improvements in sound quality.

This is really wonderful advice. Thank you. It's a lot easier to think about laying out money for a DAC, if it gets me an upgrade on the pre-amp too!