I agree with markw. Unfortunately that amp does not seem to have provision for bypassing the pre-amp stage. It may be easy to modify it if you know how to do it or know someone who can do it for you. Depending on how good the Cayin's power amp really is, it may or may not be worth doing anything else with it other than using it for it is designed for.
It all depends.
There is a proliferation of a multitude of all manor of types of Chinese amps around. Details are hard to come by.
I have found those Cayin amps are particularly popular in Germany for some reason. My German is not that good, but I have picked it a little from watching German Opera, especially Wagner.
Those amps are technically integrated. There is a discrete transformer and power supply for each channel. How they do this all for under $500 I have no idea.
Now there are three volume controls on the front one controls the front two amps, one the center and one the two rears. There is also a remote control. My German was good enough to determine that the inputs connect directly to chip opamp input amps. Then the levels are set between these opamps and the power amp stage.
Now the question, which is vital, that I can't find the answer to is how the volume controls work. If I were designing this unit, I would put the volume control not between the stages, but in the feedback loop of the op amp chips. This has these advantages.
As gain is lowered, input voltage head room is increased and signal to noise improves as gain is reduced. This is very simple design. What a Chinese designer of unknown experience would do, I have no idea.
Whether the OP can find this out from the importer
VAS Audio, I have no idea. The website is not promising, a lot of eight year old kids could do better.
However, if the amp is designed as I would do it, he will be fine, as the volume knobs will reduce gain and increase headroom.
If the amp is not deigned like that, then he needs to find out the headroom of the input stage. If it is adequate he can still go ahead, and set the volume knobs appropriately.