Is it true that mixing metals can produce different sonic results?
Yes. This goes for cables mixing metals in their conductors as well as those using connectors with different metals.
Is it best to go with one company if they offer all the different cables or is combining different okay?
There are no absolutes in audio. Going with all one company may work best for you. However, mixing and matching is fine as well. BTW, I think you mean Amadi, not Almadi.
I own a VPI table, and use a tube preamp with SS amps, however, my preamp, amp, and speakers are different than yours. Who knows? My personal musical tastes may be different from yours as well.
This is why I agree with @nonoise and @nicotico , that only you can determine what works best for you. Take your time, try different cable companies using those that offer a good return policy first, or you could try using The Cable Company for loaner pairs.
As @lowrider57 mentions, start with the interconnect between your VPI and Herron first, as this should give you the most "flavour" of the cable.
It sounds as if you are experiencing grainy sound now, with all copper cables, is this correct? If so, I would avoid silver or silver plated copper cables. I am using almost all pure silver myself, and although my system does not sound grainy in any way, silver can be very revealing. If there is grainy components in your system, the silver will reveal it more than copper.
Gold can sound a bit warmer than copper, you may want to audition the Gabriel Gold for this reason. Also, I have owned the Cerious GE interconnects, and though they claim to have silver in them, they were too warm for my system. You may want to give them a try as well.
I prefer the Amadi, but I would recommend you avoid them, as they may reveal even more grain in your system.
I have not heard the Morrow or the Cabledyne cables, though I believe that Morrow uses silver plated copper and Cabledyne uses pure silver, which I also would avoid both due to your grain issues with copper.
From my experiences, I have found silver plated copper to sound faster, more revealing, and grainier than pure silver.
Take your time, there is no deadline. You may even find some older copper interconnects, like Cardas Golden Cross, Jena Labs, or Kubala Sosna, may eliminate your issue with grain.
Good luck, and happy hunting.
Cheers,
John