Level the floor

Yeah.  At this point, cushions atop chair, mix and match, but, yeah.  One issue at a time.

I think you are on the right track. If you have some Mooney, you might go for Black Diamond Racing Carbon Fiber platforms… then use Vibrapods, or audio pucks (there are many variations) under the platforms. Short ones under the high side and thick ones under the low side. 
 

It is not expensive to get scrap granite… and have it cut. I use this for my amp… with vibrapods under and Nobsound springs above. 

You could try something as simple/cheap as hockey pucks.

I had no idea!  (I don't suppose there's a preferred brand of... no no no ignore that.)

When I was growing up playing ice hockey pretty much all the pucks came from Czechoslovakia, so if you can find some of those vintage pucks — since the country doesn’t technically exist anymore — I’d guess those NOS pucks would probably sound best.  Ehem.  I have a similar sloping concrete basement floor issue and have used generic hockey pucks when needed and noticed no sonic degradation FWIW.  That said, since you’re on a concrete slab I’d skew toward getting some granite slabs cut to size at Home Depot, etc. and just do that.  Either way I’m sure you’ll be fine.  If you’re more OCD about it you can see how much it’d cost to level out the basement floor with some added concrete.  I thought about that but my hockey past and the pucks ultimately won out.  Best of luck. 

Ehem.

[exhale]  Well done, well done.

I had no idea that custom-size hunks of granite were cheap, and/or subject to such fine specs.  See, this (and others) are just the info I came in without.

Thanks, all.