The current SSMH BOM uses some pretty high-quality parts. You could pay 2-5x the price for "boutique" parts that would only give you maybe a 5% increase in quality. Boutique parts would be things like SoniCaps or Mundorf caps, black gates (no longer available), etc. The resistors used in the SSMH are pretty much top-of-the-line as it is, so nothing to do there. You could switch out the pot for an Alps RK27 - in fact, that is probably worth the extra cost (the pot in the default BOM is good but not amazing). You could even use a stepped attenuator instead of a pot. Really, if you're looking to upgrade the SSMH you'd be better off just building a higher-end amp like the Mini-Max. You'll get more for your money that way than you would buying "boutique" parts.