Replaced stock McIntosh 12ax7a with gold lion ecc8

The filament in your new tubes is hidden behind the plates more than the ones that came with your amp. This in turn makes it (the glow) harder to see. If you see anything you know its heating and doing it's job.

I forgot to mention that if your plates are glowing, that's a bad thing and means your bias of off. You can see that if there's a glowing vertical line down the length of the tube.

Did you get matched quad?

Matched do the same thing. It doesn't matter. They're all a little different because they're basically handmade. No filament or plate or getter is the same. They all differ a little bit. How much gas and air is sucked out of them and how much gas the getter can take out makes a difference. Tubes are complicated uncomplicated things. Manufacturers just bang them out and if you need a matched set, your dealer handles it. Did you know the longest lasting on record was in an LA studio and it lasted 10 years - 80,000 hours? Tubes that generate less heat obviously last a lot longer (12AX7).

Thanks for your responses - It's starting to sound better, I did not get a matched set, I was not sure I wanted to switch permantly, now I am convinced to switch. Most of my music sounds more involving, which is amazing! The only trade off so far is some of my program gets "Busy" where the stock tubes perhaps has more micro detail with a very dry presentation. I am going through all my music collection and having a blast listening which is a very good sign. Thanks for the info on hidden plates.