Usually I don't like to correct previous posters, but jomama45 is completely wrong about the installation. First of all, you never want to backfill your wall with "impervious soil", since by definition impervious means that water will not flow through the soil and you would not get any drainage through the backfill, which will totally negate the purpose of putting in drain tile. As for using clear stone, I have no idea what that is. For backfill, you need either clean gravel, 3/4 inch crushed stone, or coarse sand.
Daniel, I happen to take offense to your mis-statements & correct all of the mistakes you made in your reply, which are numerous. First, I'll admit, I now only use Versa-Lok SRW units. I used to do some work with a Keystone manufacturer about 7 years ago, but no longer use their system. Approximately 9 years ago, I took a training seminar through Versa-Lok & recieved a 200 page installation guide that's full of section illustrations of all typical, & many non-typical applications for the system. Believe it or not, Versa-Lok has done a large amount of there own engineering on their own product, & there process seems to be vastly differnet than your approach. I think I'll stick with there engineering in this case though, as I feel they may be a little better qualified in this field. And I've set somewhere between 12-15,000 of their units in the last 15 years with no issues using their specifications. Well, I dug the manual out & every cross section of every wall section shows the the draintile slightly above the grade on the front of the wall, just like I stated earlier. Under the pipe , it states IMPERVIOUS FILL in every cross section also. That means that impervious fill is added on top of the gravel leveling pad to get the drain pipe above grade, & even pitched slightly towards the wall. At this point, the free draining stone first starts.
The drain tile goes uphill (behind) the wall, at approximately the level of the bottom of the wall. There is no need to prepare a special base for the drain tile itself, of course you need a base for the block that makes up the wall, as you indicated. The simplest type of drain tile is schedule 40 PVC pipe with the perforations already in the pipe. You simply glue it together, wrap it in filter fabric, then install behind the wall at a minimum of 2 percent slope. The pipe exits underneath the perpendicular part of the wall. You will need a T to connect the two runs of pipe. Since your wall is only two feet high, there is no issue about crushing the pipe even if you place it directly under the wall. By the way, the holes face downward.
You backfill above the pipe with 3/4 inch crushed stone, clean gravel, or coarse sand. You only need to backfill about 6 inches wide, and bring the backfill almost to grade. The last few inches you can backfill with topsoil so you can get some grass to grow over it.
Manual also states: Drainage aggregate - 12" deep minimum. Clear gravel is clear of all fines much the same I assume as "clean gravel". I wouldn't waste my time with sand as it not as free draining as stone & IMO has no benefits over stone. The cost of clear stone is minimal in terms of a retaining wall project, so why scimp? Now, assuming the minimum (of all block manufactures that I know of) embedenment of 1 course, how do you get any drainage out of the pipe if you set it directly on the gravel base? If you start with the drainage pipe behind the wall at 4"-6" below grade, it will NOT drain to the face of the wall. That is where the impervious fill comes into play, it's used to raise & pitch the drainage pipe above grade through the front of the wall.
The water will drain vertically downward to the pipe, enter the pipe, and run downhill to drain out. This will minimize pressure on the wall, although realistically for a two foot high wall, I would not worry too much about it. When the wall gets to be 4 foot or taller, you need to start worrying about hydrostatic pressure, but still, a drain tile is a good idea, and will minimize the chances of the wall moving. If you get the drain tile set properly, you can use concrete block without weep holes. If you skip the drain tile, you will need weep holes in the block, typically about every five feet.