How big of a difference do amps make?

The value of a better amp depends on the overall level of quality of your system.
For less expensive (cheap) systems, the 50% for speakers is a good idea. So a cheap amp is going to be required to fit the budget.
For better systems the amp can be more expensive.
My amp is the same cost as my speakers.
I like it. I might by better (read three times current cost) and I would use the same amp. but that is just me.
(from Magnepan 3.6 to Magnepan 20.7 And yes the 20.7 could use even better amplification, but it is all I could afford!)

For amps, that last bit of perfection can really cost a LOT of money.

So for the op, it all depends on the level of your system.
Better all around and then the cost of the amp relative to otehrs can go up.

OK:

http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

The Merlin is designed on and for tube amps and is friendly to all tube amps as long as they make enough power.

Newbee, sure, but first things first. It's just the way it is. I wonder how many amp designers would disagree.

Newbee
I decided to get rid of some of my other gear (sony 5400 and Focal 918's) to save for speakers. I just got the Merlins and I have a good lead on a pair of Thor 30's that I dont want to lose. So i planned on trying to find a stepping stone source in the 600 dollar range to save for the amps.
I am sure others may have traveled a similar path. So i am looking to hear their experience going from ss to tubes.
The Thor preamp made a HUGE impact on me so if the amps follow suite....i think im in for a treat.

McPherson, Sounds like a good plan, getting a tubed amp for your Merlins. I've not heard the Thor so I can't comment further about that but it has a good reputation. I also agree with getting the 8004, it has gotten high marks as well. In fact I was thinking of getting one for backup. I have a lot of hybrid cd's and would like to hear what SACD has to offer.

FWIW I've had three 'watershed' moments in audio. My change from a SS pre to a tubed pre, my change from ss amps to tubed amps, and finding my present speakers.

Good luck. I too think you are in for a treat. :-)