Have I Hit The Point Of Diminishing Returns?

I agree with John and Charles. In this hobby the word "dated" is irrelevant. Just listen to some great vintage gear

What John says is true, but if a particular switch or upgrade is worth it to you then that is all that matters.

Charles - yes, I think you're right. I was curious as to price/performance ratio after one gets to a certain point in the "journey." Perhaps I have a case of "upgrade-itis," but it is dissipating quickly. After asking myself the question you posed, I couldn't come up with a rational answer. I think that, in and of itself, speaks volumes.

Rlb61,
You're happy with your sound. If you read many posts on this site you'll realize some folks never seem to reach that point. Thus the constant and frustrating "upgrade" pathway.I'm not suggesting nothing can be improved upon, but what's being improved? Clear objectives would need to be solidly determined/defined.

A thought to consider: Rather than fixing what doesn't seem to be broken (i.e., "upgrading"), consider adding a high quality pair of headphones and a headphone amplifier. I find having both speakers and headphones available (Stax electrostatics, in my case) to be nicely complementary. For several reasons:

1)Having two (very different) sonic perspectives adds interest from a musical standpoint.

2)Headphone listening takes the room, the speakers, the power amp, and most of the preamp circuitry out of the picture (assuming the headphone amp is connected to the preamp's tape outputs), which can be helpful in diagnosing system issues and/or providing reassurance when audiophilia nervosa strikes.

3)Headphones, of course, can make it possible to listen when listening via speakers would disturb others in the household.

Just a thought. Regards,
-- Al