Modify tube amp or upgrade to NOS tubes

I am reasonably new to integrated tube amps, and have played around some with switching out the stock tubes for some other current production tubes - SED & JJ KT88s; Tung Sol 6sn7 & 6sl7, Grove Tube EL34 to get a sense of what differences in the music there might be. Putting all else aside - what makes more sense to those of you with more experience with tube amps if I want to go a next step - have amp upgrades e.g. resistors, capacitors, power supply; or get NOS tubes?

Thanks for any insights
You will wind up with NOS tubes anyway, so go for them first. I recommend cryoed tubes. Google
I would be inclined to go the NOS route for the preamp tubes and current production for the power tubes. I have a Prima Luna Prologue 2 integrated amp and a Prima Luna Prologue 5 power amp. I have gone with NOS for the 12AX7 and 12AU7 (Mullard & Sylvania) and have stayed with new production Electro Harmonix for the EL34 & 6550. I discovered two things along the way:

- I prefer the sound of the EL34 & 6550 to KT88 (sounds smoother to my ears)
- EH lasted longer than JJ and SED Winged C in my Prima Lunas.

Regards, Rich
Think of it this way. NOS tubes are going up in value and modifying a piece will reduce it's value
I agree with others that the first approach should be NOS tubes for the preamp. You'll get the best bang for the buck with no risk of losing money.

But if you're into DIY projects, you can further improve the sound by doing some simple upgrades that don't cost much. I don't suggest doing this willy nilly. Get in touch with someone who knows what they're doing and has a good reputation. I highly recommend Jim McShane. He's well known at Audio Asylum in the Tube DIY forum and well respected for his amp upgrades.

He helped me tweak a pair of Antique Sound Labs for a ridiculously small amount of money (less than $200). We obviously didn't use boutique caps like Mundorf or V-caps, but the result has been very sweet. My top end is smoother, less fatiguing and more enjoyable. Anyone who knows what they're doing will need a schematic. Jim focuses on the power supply as much as the coupling caps and signal input caps. He's a fan of using film to bypass electrolytics, Vishay fast diodes in the power supply, Russian KY40 PIO for signal caps, Multicaps for coupling, and carbon comp resistors in the signal path.

Some will argue that it's never wise to tweak like this, but the fact is, most equipment was built using inexpensive parts to keep the price down. For me, a couple of factors dictate whether or not to go this route:

- I don't mind losing the money I put into parts. DIY upgrades rarely, if ever, increase the value of any equipment.

- Will I reduce the value of the equipment? It doesn't make sense to do this to an Atma-sphere, VAC, VTL, Canary, etc. In my case, ASL is largely considered bargain gear which is ripe for tweaking.

- I plan to keep the equipment for a long period of time. Why do it otherwise? There has to be a positive foundation to build on. You can't make a silk purse out of a sow's ear.
I highly reccomend Jim McShane for sure. I purchase all my tubes from him.
If the integrated amp is a piece you like and want to keep and as Mingles said it may add value to your piece of gear and improve the sound I say make the investment. But I caution you that all mods or parts swapping won't make a poorly designed circuit better. Make sure the person you have doing the mod knows your product, has a schematic for the amp and understands the properties and sound properties you are looking for. I haven't dealt with Jim in this manner yet but I would ask him to see if he is interested or thinks it is worth it. He is an honest guy so trust his opinion.
I have a Dynaco ST 70 with a Sound Valves input board. But all the other Mods were Done by Tom Tutay of Transition Audio Design in Florida. I have had this ST 70 for over 10 years and will not part with it. I have invested allot of money into it as well and I could never sell it for anywhere near what I paid for it. But I'm happy with it. I listen to every speaker I have with that Stereo 70. Speakers like Living Voice Auditoriums and Avatars, Magnepan MG12, Spica TC 50's and soon TC 60's, Von Schweigert (spelling),Aurums, Athenas, Tannoys, Wharefdales, Alon I MKII, and some others. It was a worthwile investment. Find someone you trust to do the mods and as I said knows if the mods will make a difference. Beware of the parts swapper. You can do that on your own.
Talk to Jim about the tubes. I think if the circuit has been designed properly finding tubes that work for you will do the trick. The power supply is always a compromise for budget gear.

Cables can sometims make a difference too.
Thanks for all of your input - will try NOS tubes first