My stereo receiver is a little too bright. Can a cable help me out?

Changing cables WILL affect the sound. Why do people STILL naysay this ....? Some are  on the bright side and some are duller than typical.

I have an Oppo 105D and do not use it to stream music as it can be a bit harsh, so that would not help, and your Roku is a pretty low level device too, so you are not helping your system much

If you love vintage gear, trade your Pioneer for a Sansui AU --17. I would consider an AU-717 or AU-917 integrated amp. They sell for about what your SX-1050 is worth. I used to sell both lines, in the day. The Pioneers always sounded too bright, to me. The Sansui AU---17 were as good as vintage aficionados say the are. Later series AU---19 and such sounded good, but started using cheaper internals, especially the power supplies. 

I have made the mistake of spending a lot of money re-building much better preamps (Conrad Johnson PV 5, PV 10) from the 80s and 90s because I felt I couldn't afford new. My wife got fed up, I saved some money and now I'm very happy listening to music daily. Nothing from thirty or forty or fifty years ago sounds nearly as good as the better stuff made now. The best engineers actually have learned a few things in that time. I did hear a pretty nice Spectral amp that was thirty years old, but the new stuff is MUCH better. On a budget, get a new Arcam. 
As far as cables go, many are bright or forward to give the short term illusion of detail (I'm looking at you, Nordost). The only manufacturer who references live classical music in cable design is Transparent. They are the most natural sounding at every price point. Don't be dismayed by the prices of their top stuff, they have the same design criteria all the way to their most affordable.
Cut your losses and move on - you will be happier.

re older transistors. re old 70’s gear.

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transistor heyday regarding actual sonic quality, was in the 70’s. the mid to late 70’s.

scratch the opinion of any seasoned designer of audio gear and they will tell you the decade was the 70’s to the 80’s for design and publishing of high grade audio oriented transistors.

since then, we’ve had to make do with transistors that are ’accidentally’ good for audio, as opposed to transistors being designed from the ground up, specifically for audio. Which was the case back then.

so, in essence, transistors good back then? yes, 100% ...design was mature, it’s gone backward for audio since then.

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parts quality. Switches. Back then? superior to most modern equivalents that are twice the price of back then, if not 10x the price of back then. (materials costs, environment, economies [inflation, etc], etc...all of it increasing dramatically in modern times compared to back then)

they need to be stripped apart and have contacts mechanically cleaned. Which can be done with the older switch, but not with the new inferior one, in many cases. Some switch companies still make the older higher quality switches, like Alps does, but they cost and they really aren’t the same build quality as back then.

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circuit design? this is a bit trickier, but in most cases, most modern and currently used circuit design tricks of today, existed back then, so the deficiencies in design and build in that area are not really there or really a problem. Some of the more modern cutting edge or even ’long term audio design thinking’ tweak tricks can be mechanistically added on to the older circuits in the older gear.

The idea of the new circuit and so on being better, exists as companies have to sell you the new in order to be alive and exist, so expect them to convince you that all is new all the time --- when in fact it is not. Not at all.

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again, parts quality? Resistors and capacitors? these can be modernized and upgraded, and then the gear can exceed most or almost anything you can buy new today, that is transistorized.

Like a old Camaro or corvette, etc... being refitted to take on a modern carbon fiber bodied racing F1 Ferrari, on the track.... and be dead even and..in some minds... even win. Imagine that.

Granted, this is a tall order and the labour levels are insane. This is classic car complete tear down and rebuild ’insanity’ territory, but as done on audio gear.

(link is for the image of a ’properly’ modded out NAD3120, which, when finalized [finally finished], might [and generally will] exceed the sonic qualities that most seek to experience from a modern $5k integrated.)

https://www.canuckaudiomart.com/forum/viewtopic.php?f=22&t=45215&p=708943&hilit=nad+3120...

@teo_audio is right.  You need to give it time to break in.  It will take around 50 to 100 hours of operation to fully break in.  Secondly, a receiver with 40 year old caps typically sounds pretty dark.  So there is an added contrast.  Our shop has a lot of experience restoring these old gems.  We however, tell the customer the cost before even starting work and let them make the decision before spending any money. 

If you are still unhappy with the performance after break in, the good news is that a restored SX-1050 is worth around $1000.  In contrast, a used AVR will drop in value faster than anything.