VAC 300.1/300.1a tube rolling

Does any one have experience with using 6550s in the VAC300.1a?

I just bought some NOS Tung-Sol grayplate 3-hole 6550s for use in my unit. I understand from Kevin Hayes, who as always was highly knowledgable and helpful about his great products, that the 6550s, which are rated for less voltage than the KT88s, are a little shy of what is needed by the amp. That said, he felt I should be fine, assuming the tubes are properly tested and in good condition, because the build of these NOS tubes is very good. I have heard that this may not necessarily be true of the new production 6550s, which of course I don't plan to use.

I am wondering about the experience of others with 6550s in general in this amp, and also if any one has experience with Tung Sol 6550s in particular. I love VAC's sound and have experimented successfully in my Renaissance Mk3 preamp as well as in the input section of my 300.1a with NOS Tung Sol and Raytheon tubes. I only decided to experiment in the power tube section because I have found myself to like the sound of these NOS tubes elsewhere and figured that it might be a good experiment. I had not appreciated the point about 6550s not being a literal drop-in for the KT-88s, so I am a bit apprehensive about that.

Kevin also told me that he believes the family of tube sound likely carries over from one tube type to another for the same manufacturer because of production values, metallurgy etc. This seems to makes sense to me too given my experience with 12au7s, 12ax7s and most recently 6sn7s.

Just wondering if any one has some useful experience to share or light to shed?

Thanks in advance.