I'm Wes from Tisbury Audio. I've tried and tested most attenuators on the market, so I thought I could offer a bit of helpful insight.
All stepped attenuators will have a detended rotation as this is required to ensure the wiper settles correctly on each step. However some are smoother than others.
DACT & Goldpoint: These both use the same Elma switch, so both feel the same. The rotation is noticeably stepped, but switching between steps is smooth and effortless. These are both serial types (I'll cover different types and their impedances below).
Dale attenuators: These have a much stiffer rotation. They offer brilliant sound quality, channel matching and price, but the stiffness is a real downer. You can get them in serial, ladder or shunt configuration. I'd only ever recommend ladder for these.
Chinese SMD attenuators: These are available on eBay from gigawork and lasercollection for about $10.
I actually really like these. They have by far the smoothest rotation of all the attenuators. You can feel the steps, but they're very smooth.
They're serial surface mount resistor types, the same as DACT & Goldpt. They sound great and have good channel matching.
Because of the price, there have been concerns on other forums about build quality, but I've not personally had any problems. Plus they're so cheap you can just buy 2 or 3.
Seiden/Shallco: These use a spring loaded ball against a cog to step the rotation, so as you can imagine the rotation goes 'clunk, clunk'. They're very expensive, and the main advantage over DACT/Goldpt is that they can be made in any configuration and with any resistors.
Different types and their impedances:
Ladder: These can give the most consistent input and output impedance if the resistor values are chosen correctly. They can sound fantastic, too. However, they are usually the most expensive, and obviously the resistor values need to be correct.
Serial: Although 'serial' means passing the signal through multiple resistors, as A9X-308 said this does not effect distortion and these are some of the best sounding attenuators you can get.
These will have the same impedance behaviour as a pot. Input impedance is fixed at the value of the attenuator. Output impedance will vary slightly, with the worst case scenario being approximately 1/4 the attenuator's value.
Shunt: You are quite right, the shunt resistor does indeed affect the signal. In fact, it's quite plausible that the shunt resistor is more critical than the series, as the signal seen by the amplifier is effectively the voltage across the shunt resistor. This was very well covered over on diyAudio (search "shunt attenuator myth").
They are basically a gimmick. The reason they've caught on is that they can be constructed fairly cheaply, while appearing to have high performance. But the companies that use them conveniently don't mention this comes at the expense of impedance. Output impedance will vary from 0 to the value of the series resistor + source output impedance. Even input impedance will vary, although this is usually less of a problem.
Also I think you mentioned your amp is 25k in. In which case you'll be wanting a 10k attenuator.
And if you do go down the DIY route, for the love of God don't over tighten the fixing nut on an attenuator/pot! It can be an expensive lesson to learn. They have a little nub to stop them from rotating, so use that and keep the nut tension fairly light.
Hope that helps!