DAC/ Streamer - to upgrade or not to upgrade?

Dear Audiogoner:

With encouragement from Audio-Union , I like to seek further advice from those who are knowledgeable on streaming / DAC in accessing digital music. Currently I am connecting Roku 4  (accessing Tidal MCQ ) sending through its only optical out , to an optical-digital converter, then using  coaxial cable ( Siltech classic Anniversary Series model HF pure silver-gold Coaxial cable ) to a  vintage DAC (Altis Reference) at 48KHz. While the optical to digital converter may diminish the SQ, to my surprise the SQ from my streaming using above connection is only slightly worse when I compare to playing same CD music (whether classical, jazz or popular) using my CD transport(CECTl0) with Siltech cable connecting to the DAC. I had previously raised a question at this Forum about replace the Altis DAC which was positively supported by  some Audiogoner. I then tested Black Note DAC30 Tube Model in place of Altis and I found Black note was only slightly better in SQ to my ear. (By the way, my other equipment are Doshi Alaap V.2.1preamp, Lectron JH50 amp, and Minent  M7 speakers). I hope fellow Audiogoner can provide  advice on the following

1.       In order to improve CD  SQ more noticeably in my system when playing CD with CEC, what DAC would you recommend as the next step to replace Altis?

2.       On streaming,  I wonder if anyone has experience with Roku4? What streaming device (as next step upgrade) would be noticeably better when connecting to Altis? Would I need to replace both Roku and Altis to get noticeable improvement in streaming SQ? If so, what would you recommend?

Thanks a lot.

Sincerely

Moses 189