Tweaks to Add A Little Treble Emphasis...

Yes you are right try silver ICs. I like the brand - "Home Grown" model- Silver Lace.
Also try brighter sounding 12AX7s instead of your 5751s. Some examples are Ei Silver Bullets which are now NOS. Sino C9s if available are remarkably clean. As mentioned the Siemens, and I would add, Telefunken, although the Teles are less treble oriented. Both are good if cost isn't a major obstacle. Getting real Siemens 6922s is difficult and very expensive. An alternative is the US made white label Amperex which are nice and clean. They are frequently found as OEM for equipment manufacturers such as Hewlett Packard, Beckman etc. They are usually a bargain.

Onhwy61 has an excellent suggestion.

How old is your aesthetix calypso? Maybe it is time to retube. You can't use interconnects or power cords as tone control if the root cause is aged tubes.

What kind of speakers are you using? Also, how is the room furnished and how much soft surface is there? Too much thick carpet, stuffed furniture, and especially heavy drapes and wall hangings can overdamp a room, killing the treble unless you are directly in the sweet spot.

Also, 12AX7s would definitely brighten up the sound compared to 5751s. Also, better power cords won't specifically add treble, but the right ones make everything more clear and dynamic, but the clarity applies to all frequencies.

Do you want to play with your crossover at all? If yes, you can reduce the series tweeter resistor value by .5 or 1.0 ohm. That will result in increasing the output of the tweeter by a bit.

Enjoy,
Bob