Your photograph shows a potentiometer with a loudness tap. This means the potentiometer needs to have 4 pins per channel, so 8 pins in total. The one you linked is not suitable, because you can not conect the loudness curcuit which uses the tap to cut off the "loudness equalization" when the wiper passes the tap position.
The location of the loudness tap is not necessarily 50% of the volume. This depends on the design of the amplifier. Potentiometers were sometimes custom manufactured for the various audio companies. These days very little is available to substitute some of the obscure potentiometers that were used.
Be aware that there are several types of "tapers" which is the resistance variation with the rotation of the potentiometer shaft.
@leesonic said that the NAD3020 uses a Logaritmic taper. The one in your photo is marked with a B but i am not sure if in the the manufacturers denomination was different from the one used now. Currently the manufacturers use a "B" for linear and "A" for the so called Audio taper.
If I were you , I would try to salvage the original potentiometer. If this is not possible then if t least one of the two channels is still working acceptably measure the resistance of the Loudness tap relative to pins one and 3 so you can try to fin the best match.
For measuring the taper of the potentiometers,
please take a look at this domument.
Finding an Alps potentiomter with loudness tap these days is nearly impossible. There are lots of china made copies that are really bad because they use a resistive divider made of SMD resistors, so it is a stepped attenuator. This is in principle not an issue, and you could in principle modify the position of the loudness tap or even add on eif it was not there. But the problem is that they are manufactured with a Printed circuit with the rotating contact "wiping" on a standard printed circuit set of contacts (albeit gold plated). With little use, the gold plating wears out, and then the copper underneath is expossed. Copper oxidizes and the potentiometer soon becomes noisy (scratchy as they say). Avoid these all together, and please do note that they are all encased in a copy of a 27mm Blue Alps body (RK27 model). These copies are some times called RH2702. Here is how they look like:
Note the erased brand on the second row.
There are also copies of the alps that have real carbon tracks inside, and they make decent substitutes but these are always in the 6 pin variants (no loudness tap). They look like the one you linked but if you observe inside they have a carbon track instead of a set of smd resiators and rotary switch.
From my personal.observation I add that
the true Alps potentiometers always have Serif in the font used for Alps. Some opies do too so you have to be careful. The pots that have no Serif font are to be avoided as they tend to be the worst copies.
You can use potentiometers made by Bourns but they are hard to find. The next brand to consider are the Alpha Taiwan. They make 20K potentiometers both linear (B20K) and logarithmic (A20K), and they make them in tandem (dual gang for stereo) with and without the loudness tap which is always at 50% of the rotation. So the tap is not in the 50% resistance vaue in the case of the A20K potentiometer. I can try to dig out a photograph of these if I can, since I somewhere have the 50K variants.
Mouser carries this brand but they do not have all the models. If you inquire them they may be able to help you finding it.
They are also available on epay:
A100K A type with loudness tap
B100K type with loudness tap
You can surely find one, they are not as good as an original Alps potentiometer, but they are decent and will last a lot longer than the chinese copy i described above.
