When were the best tube amps made?

Sometime in the future.

I second PrimaLuna.  My EVO300 integrated tube amp is wonderful when upgraded with Golden Lion KT66 tubes  

With a single ended tube amp it is easy to build your own. But to keep it linear you can't push a tube very hard. You need heavy duty transmitter tubes. There is a WAVAC SET that sold for $350,000 that you can build with under $2000 in retail parts. It runs 833A triodes at about 1000 Volts which will make these tubes last for many years. Hammond makes a transformer more than robust enough and more robust than any bigger name or vintage transformers and Lindahl makes an input transformer for the grid suitable for a 45 SET to drive the 833A. 

This gets you SET purity you can use for less sensitive speakers such as planar magnetic speakers which have a purity that is difficult to approach with cabinet speakers. 

Transformers are the key element for both SET and push pull.  Even for OTL the power transformer is relevant.  Likely the best commercial winders were in Japan in the late '90s, companies like HIrata Tango and Tamura.  I think back then there was a focus on sound and quality rather than volume and profit margin. 

Likely there are currently some good boutique winders, and maybe Western Electric were good (never heard an amp with these).  Useful to distinguish between collectable and listenable - not sure where the WE falls.

I certainly do not equate 'high tech' with 'great sound'.  Frequently the application of high tech results in additional complexity and worst sound.  Hard to make the argument that the more you process the signal the better and more realistic it sounds.....

@mulveling @sns  Yes, I chose an AR SP-14 over the 9. I did not like the 9.  The 14 had some superior sound advantages.  The SP-10 was my favorite and the SP-8 my second favorite of the era.  They had big, bold, full bodied sound (like a Fisher 200? preamp that I briefly owned).  Comparable to a McIntosh MC30 full range.  

My neighbor has an AR LS-28.  It sounded thin and blah until he installed 6H6/6N6 tubes for the 6H30s.  Wow, fantastic improvement.  I would own that pre-amp if I didn't have my own excellent pre-amps (multiple systems).  

As to great sounding, powerful modern tube amps that are still reasonably priced is the EAR 890.  I just purchased another one inexpensively.  Paravacini knew transformers and circuit design, regardless that they are not considered state of the art and used ordinary audio parts, no special cables, fuses, tubes or footers (I use them all on the 890 except the tubes remain the same).  Zesto is copying his designs and I have not auditioned them to compare.  That's flattery and could be a trend, using common tubes in non-ultralinear designs.  I considered the VAC iq200 but the EAR is just better sounding and cheaper.