I have not heard any of these plinths but I can say that Albert Porter is a class gentlemen who I wouldn't hesitate to buy from. I have no doubt his plinths are outstanding. Steve Dobbins I know of only by reputation but I don't think you can go wrong there either.
Another "brand" of plinths you might consider is OMA slate plinths from Jonathan Weiss. If your from the northeast they have a showroon in NYC. At the least you would get a chance to hear an SP 10 in a proper plinth. They also make a SP 10 where the individual components are taken out of the metal casing and individually mounted in a 200 pound plinth. You can check out their plinths, etc. on their website.
Another "brand" of plinths you might consider is OMA slate plinths from Jonathan Weiss. If your from the northeast they have a showroon in NYC. At the least you would get a chance to hear an SP 10 in a proper plinth. They also make a SP 10 where the individual components are taken out of the metal casing and individually mounted in a 200 pound plinth. You can check out their plinths, etc. on their website.