Audioquest Niagara 1200


Can I plug my Preamp to one of the high current slots better soundstage or it won't make any difference?



Only your ears will know for sure. Why haven’t you listened both ways? We cannot hear your system with your 👂’s!
It will still offer some filtering, though not as much as the other outlets. That may not be a bad thing as what filtering you do get may be all you need. 

Having an integrated using one of two H/C slots I had the other free to use. My SACD player was supposed to go with one of the remaining outlets employing more filtration. I tried both high and low filtering outlets and couldn't, for the life of me, hear a difference. 

Using both sounded equally great.

All the best,Nonoise
I've listened both ways with my 1200 and ended up plugging my preamp into a high current slot.  My VAC preamp has a robust separate power power supply that I feel benefits from the high current.
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It definitely won't "hurt" the preamp. There is simply less filtering on the high current outlets so that they can deliver more current. I actually prefer those outlets over the others.