Schitt Audio Still in business???

You can leave amp off if you want. As long as the input signal from source goes to the Yggy, and the cables are connecting its output to the pre/amp you are fine. You just want a signal going all the way thru the new device.
As a side note, don't ever run a tube amp without a speaker connected or you could damage the amp. Cheers,
Spencers

Thanks Spencer .  I will commit that last point to my hopefully long term memory.

Ideally you’d want to have your amps on also actually. You want the signal going all the way through every piece of solder, wire, capacitor, etc. The DAC has small amps/power for the output signal that you want going through it’s paces during break in, and out through the last bit of circuits and wires and such. It’s not just the actual DAC chip that is the only thing getting broken in. I mean you have an entire huge analog section that is like half the DAC after the DAC chip that needs to be broken in and have a signal going out through it to some amps. While it will mostly break in still with the amps off I feel I got better results with them on. Without the amps on I don't think the signal has any place to travel and be used and is not actually traveling a signal through if there are no amps on to use that signal. I’d alternate between leaving them on and having them off since I didn’t like having my amps on constantly.

But yeah this thing really does need 100 hours or so at least to break in. Mine was dramatically better after about 100 hours. Supposedly it takes up to 400 hours to be 100% done some have said. Seems minimal so far after 100 though. But I think I read that others have said it can go in spurts even after the 100 hours.

It’s definitely an incredible DAC. Mine benefited by using the USB regen. The regen still makes a big difference even on the yggy.

Official Answer from Shiit responding to my email

In our opinion, both inputs offer similar performance, and no input needs to be used for run-in—just turn it on and leave it on continuously.

Personally I don't believe that you could just simply turn it on with nothing attached and have it burn in completely and thoroughly. I mean yes you'll get some burn in since parts of the DAC will be powered and moving some electricity through it. But not totally and 100 percent of the DAC or as quickly. I'm sure it will have improvements but there is no way to break in parts that aren't even used if it's not running a signal and current through it. I'm a firm believer that a current and signal needs to run through each and every milimeter. Up to the very tip of the outputs, anything that carries the signal. Every piece of solder etc. Just like you couldn't have your amps off and expect your speaker wire to burn in without a thorough strong current running through it. I think the DAC will improve some but will take longer as the rest breaks in each time you actually go to listen to music. But I guess try it which ever way you want. I go for the way I think is going to be the fastest so I can enjoy it's sweet perfection the quickest. At least it makes me feel better in my mind :D