It always amazes me when people comment on products they do not own or have not heard. I have the Mullard 12AX7 reissues and while do look like the Sovteks the sure don't sound like them. They have a much smoother more extended sound and are not strident like the Sovteks. I use them in a Cayin A70T and Cayin MT12A (an EL84 amp that I have not seen in the USA). The A70T came with EH 12AX&'s and MT12A with Cayin branded Shuguang 12ax7's. In both cases the Mullard re-issues were an improvement.
I also have a set of the Tung Sol 12AX7 re-issues and the pair I have are very low gain,the sound seems very recessed and they are a short plate design as compared to the long plate Mullards.
As to output tubes I have not had a chance to listen to the GEC Gold Lion re-issues but I do have two sets of the Tung Sol 6550 re-issues one in an Audio Research CA50 and a pair used in my Antique Soundlabs MG SI 15 DT single ended amp. In both cases (they replaced SED6550's in the AR and Valve Art KT88's in the ASL) they were a significant improvement. To my ear they are very similar to the original GE6550's as I still have one Quad I use a reference. I hope this gives you some help in making your choice.
I also have a set of the Tung Sol 12AX7 re-issues and the pair I have are very low gain,the sound seems very recessed and they are a short plate design as compared to the long plate Mullards.
As to output tubes I have not had a chance to listen to the GEC Gold Lion re-issues but I do have two sets of the Tung Sol 6550 re-issues one in an Audio Research CA50 and a pair used in my Antique Soundlabs MG SI 15 DT single ended amp. In both cases (they replaced SED6550's in the AR and Valve Art KT88's in the ASL) they were a significant improvement. To my ear they are very similar to the original GE6550's as I still have one Quad I use a reference. I hope this gives you some help in making your choice.