usb, spdif, or usb to spdif converter

Which is better probably depends upon your hardware.  The SPDIF is more generic, I think, so it might all depend on how your DAC handles USB.  DACs convert USB to I2S internally, so that if your DAC does this well, no need for external USB to I2S device like the Singxer.  

Ultimately, a sound test is best.  However, for example, if the USB board in the DAC gets its 5V internally, as opposed to through the USB cable, it is likely the best way to go.  This is something to consider when buying a new DAC.

The sony blu ray players are an okay transport for digital coax s/pdif, but they are certainly not the best.  To be honest, most of the bluray players out there are not the best S/PDIF transport.  If you want a really good player/transport for both movies and music, look for a used Oppo 103 or 203 player and then upgrade it with a linear power supply from oppomod.com.

Also, your coax digital cable will affect the sound quality you get as well.  Make sure you have a good one.

Otherwise, you can run with a computer if you want.

One more thing.  If you are running the ifidac using coax spdif, you may want to look at an outboard USB linear power supply, like the LKS:

https://www.ebay.com/itm/133687208763?hash=item1f2060eb3b:g:EVwAAOSw-4BXX8qS

Unless you charge the Ifi DAC separately and then unplug the usb cable before turning it on with coax input.

Thank you. After reading the last 2 posts my gut tells me that my computer is a better transport than the sony BR player. Also I am playing rips from the computer's hard drive so that may partially explain the difference.  I power the idsd dac from its battery, not the usb cable, and it does sound better that way.  However, I have heard many times, on this forum and others, that the usb interface should never be inside the dac - hence the reason for my question regarding the usb to spdif converter. My dac is around 7 years old and I'm due for an upgrade - any recommendations new or used up to 1,000 that would best the idsd? One that was recomended was a used monarchy audio M24 but they are hard to come by.       

"the usb interface should never be inside the dac" ??? 

That's news to me.  They are on most of the acknowledged great DACs. 

Ah, I was thinking of mentioning another alternative, but I did not because of the complexity.  However, since you stated that you are actually ripping your BD/DVD discs onto computer files and then playing them, there is another option.  (ripping is usually too complex a process for most people).

You can look at one of the high end media players, such as Zapitti, Zidoo or Dune.  You would just take your ripped movies and copy them to a SSD which can be inserted into these media players.  These will likely have better video quality than your computer.

The Zappiti Reference actually has a built-in DAC uses ESS saber dac chips and full linear power supplies for everything:

https://www.zappiti.us.com/product-page/zappiti-reference

It's way over your stated budget, but it's an all-in-one box.  The DAC may actually have more resolution and detail than your ifi (it's difficult to tell).

Another option with high-end power supplies and dac is Dune Ultra 4K:

https://www.dune-hd.com/collections/frontpage/products/dune-hd-ultra-vision-4k

Dune has a lesser model with smaller linear power supply and uses AKM dac instead of ESS, but it's only $699:

https://www.dune-hd.com/collections/frontpage/products/dune-hd-max-vision-4k

If you want to try one of these media players as a digital coax transport only, the cheapest way to go is a Zidoo Z10pro:

https://shop.zidoo.tv/products/zidoo-z10pro-new-arival-rtd1619dr-media-player-4k-uhd-android-9-0-dolby-vision-support-hard-drive-up-to-14tb-2g-32g

Then add a good external linear power supply for this such as Farad Super3 (or Keces if you have a budget, or even a cheap china ebay Teradak).