I'll suggest something wacky, in case you don't want to or can't modify your pots. How about connecting two standard potentiometers together with a mechanism, such that twisting a knob moves one pot, and twisting the knob in the other direction moves the other pot?
One possible way to do this is to have two linear pots with their grounded connections next to each other and electrically connected. Imagine a piece that goes between the two pots and it connected to both pot inputs via an elastomer (i.e. just wrap a rubber band around the pieces). When you move to the right, one linear pot will move freely, while the other is stuck against the hard stop. But because its connection to the input you're moving is elastic, the connection just stretches and doesn't risk breaking the pot. When you slide back in the other direction, the pot that was originally able to move will bottom out against its hard stop, while the one that was previously bottomed out will now move freely.
It's a little corny but I thought I'd throw it out there anyway. I can post a sketch if you like. :)
What you are missing is that the pot is wired such that it produces a ratio of the input signal with the ratio varying from 0 to 1 accross the full sweep of the pot. This is why the 5 kΩ and 10 kΩ pots resulted in the same full volume.
The pot achieves this by being a resistor divider. It does not work by adding resistance in series with a signal. A resistor divider looks like this:
The output will be R2/(R1+R2) of the output. In the case of a pot, R1 and R2 are one continuous resistor with a mechanical wiper picking off OUT at some point along this resistor. The three pins of the pot are the two ends of this resistor and the wiper tap. Therefore, R2 will vary from 0 at no volume to (R1+R2) at maximum volume. Also, R1+R2 is always fixed, and is the resistance value specified for the pot. In your "5 kΩ" pot, for example, R1+R2 is 5 kΩ, which is the value of the physical resistor that the wiper slides over.
At half volume, for example with the 5 kΩ pot, R1 and R2 are each 2.5 kΩ. OUT is half of whatever signal is applied at IN. Note that since everything is ratiometric, you get the same answer whether the total pot resistance is 5 kΩ or 10 kΩ. This is why the volume levels didn't change.
The total pot resistance does matter in other ways to the driving circuit and whatever is using the signal at OUT. The 5 kΩ pot will require whatever is driving IN to provide twice the current than is necessary with the 10 kΩ pot. You don't know what exactly is driving IN and what its design constraints might have been, so it is best to replace the pot with one of the same value. It seems you got lucky in that whatever is driving IN can cope with the 5 kΩ load, but it could just as well have started clipping, otherwise distorting, or have the frequency balance different.
The crackling and the fact that you got sudden jumps in volume were due to the old pot being worn out. As pots age, dirt and oxidation accumulates on the surface of the resistor where the wiper slides over it. The resistor itself can also get worn down due to mechanical abrasion by the wiper. The wiper sometimes making good contact and sometimes not can sound like crackling, especially when the pot is being turned. Dead and worn out spots on the resistor can cause sudden jumps. These are all common failure modes of pots.
This is one area where construction quality makes a big difference. El-cheapo pots wear out a lot faster and may not be as well sealed against dirt or the materials are more prone to oxidation. If you want long-lived mechanical volume controls, you have to spend the money on good quality pots.
This is also one reason these things are done digitally nowadays. You can get a microcontroller to handle the audio stream digitally for less than the price of a top quality volume control. The digital multiplies inside the micro don't wear out, crackle, or drift over time.
Best Answer
Normally a potmeter has three connections. The middle is the wiper which moves over the resistive path, from the left pin to the right one when rotating clockwise. Swap the left and right pin and you should be OK. Important: be sure that it's a logarithmic potmeter instead of the regular linear ones.
edit
I read that the resistance increases if you rotate clockwise, and that you want the reverse. I don't know what the surroundings of the potmeter look like, but for a volume control you normally want to increase the resistance (between the left pin and the wiper) when rotating clockwise. If for some reason you want the reverse you don't want a logarithmic potmeter, but an anti-logarithmic one, which is much rarer.