Do be aware that the Digital Source you have is going to be very difficult to surpass with a Vinyl Source.
Also the DAC you are using will from my experiences, substantially benefit from being seated on a Support Structure that is very suitable for a TT support. Valves in a device, be it a Source Ancillary, Pre-Amp or Power Amp, benefit from a support structure that works for the improvements in management of transferred energies.
The E88CC Tube in the DAC, will 'if exchanged' for a late 50' - early 60's Tube produce a quite unique DAC, that will also transform into a extremely impressive performer.
I myself have gone down this same route with a DAC and Phonostage.
I have also met a few who have gone down this same route, and when discovered their preferred Valve from this era are completely sold on the exchange.
I am yet to personally meet an individual who has changed to E88CC's from said vintage, who come out of the experience believing there is another option that can better them, or wishing their earlier used Valves were reinstated.
The additional attraction of your DAC, is that the Valve options are not limited to E88CC only.

