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.
