What is the latest greatest USB add on device?

If you don't mind spending the money there is NOTHING that can touch the equipment below.

There is nothing actually even close but like everything that is outstanding it is expensive but IMO worth it.

If you want best bang for buck get the Uptone Audio ISO Regen and power it via Ultra Cap LPS 1.2 but if you want the best then on read below. 

Actually the best is to dump USB and jump on the Ethernet train as it is leaving USB in the dust but that is another topic and you asked for USB and this combo bellow has no peers.
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SOtM: tX-USBultra special edition: $1,470.00
- with external master clock input add (50 ohm): $300.00  
- 9V
https://sotm-usa.com/products/copy-of-tx-usbultra-regenerator-1?variant=12385178550351

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sCLK-OCX10 Reference Master Clock: $3.500.00
https://sotm-usa.com/products/sclk-ocx10

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SOtM: sPS-500 Power Supply: $500
- Y cable stranded 7N UPOCC Copper 30cm: +$300
https://sotm-usa.com/products/sps-500-power-supply?variant=40218821772

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If you don't have a high quality USB Server option then get this one below also and pick another PS.

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sMS-200ultra Neo Special Edition: $1.400 
- with ethernet switch mode: + $300
- with external master clock connector add (50 ohm): + $200
- 9V
https://sotm-usa.com/products/sms-200ultra-special-edition-1