I have used the Mingda KT88s and They are Shuguang basic KT88s, the standard Chinese tubes. All KT90s supplied new are made by Electro harmonix in Russia. I find the KT90s good reliable valves sounding a little brighter than most 88s, I like them! The stock chinese 88s are fair valves for the price but I hear of reliability issues?
As for KT88s, depends on your amp? I have a Mingda built amp that came with the EH 88s and they show up as a well balanced reliable valve at a very good price! Bang for bucks comes, for me, with the JJs, they come alive with my amp! With other 88s I have had I found the Genelex to sound very balanced, but unreliable and for that reason I no longer use them.
If price is not a problem I have found the Treasures to be excellent, i have used a quad for 4 months, no problems so far and the nearest tonally to the "Jewels in the Crown", Original GEC (Genelex) NOS KT88s!
I am lucky to have a large stash of the GECs, my working Quad are 30 years old, been in service for over 15 years now and are not showing any signs of wear at the moment. imho. no other KT** comes near to them! BUT you have to pay a lot of money! At the moment NOS balanced matched pairs are appearing in the UK and sell for £600.00. from trusted valve sellers. Worth the money? thats up to the buyer to decide!
I have played with the new KT120s and if your amp can cope with the extra heater current draw and other parameters that are different then they do a great job for the bucks! Just remember they are NOT a direct replacement for the KT 88/90.