Cheetahs or heavily discounted Colorados are both good choices. The Colorado is warmer and more laid back, but has more 3D imaging. Cheetahs are more neutral. For 500 bucks used you may also be able to get a Niagara, which I like better than either -- though that impression is based on only 1 pair from SUT to phono stage, vs. multiple pairs and multiple usages I’ve had of both Cheetah & Colorado. On all of these you should be able to resell with minimal losses, but of course beware of counterfeits -- I’ve seen a counterfeit Sky pop up a couple of times here, recently.
Long ago I ran an A21 with Legacy Signature III. The A21 was a great amp and pretty much got the most out of those Sigs that they had to offer -- close to even the JC-1 monos I acquired later. I think the A21 has a nice sonic signature (the right amount of warmth for a SS amp) that would work well with silver cables.