After finally getting the chance to listen to the example clip, I agree, that's THE SOUND. These guys did almost as nice a job as the Moodies in capturing that distant hugeness.
And, since I acquired mine, I've come to realize that what is making that sound for me is a couple of things, and may not necessarily be confined to a single amp/speaker/mic/medium combination, but could be fairly repeatable under a lot of different circumstances.
First, as I observed once before, one thing that I think makes Mellotron instruments, with particular reference to the classic 3-violins sound, so unique is that each 8-second strip of tape is a complete world in itself. Three instruments in a certain physical relationship to each other that is constantly moving and evolving, developing all kinds of minute phase differences, all being shaped by the sound of the room that they were being recorded in.
Then put that sound in a BIG room and play it real loud with less than hi-fi audio equipment, and add some artificial reverb to it that is also less than hi-fi.
That yields the original playback sound, the ambience it already possesses, the artificial lo-fi reverb ambience, and the natural real reverb and echo characteristics of said big room.
Then record it all from a fairly good distance, preferably at least twenty or thirty feet, if not more.
The final piece of the puzzle is to tune the Mellotron down by 1-3 cents or so, just enough to give it a little bit of melancholy flatness in pitch.
You get the three ambiences playing against each other, the distance, the thickness of sound, the individuality of each strip of tape, and that little bit of sad wistfulness that the flat intonation provides.
You will find that the sound is much like the recording. Depending on speakers, mics, and all the other twiddly bits of equipment, it will flavor the sound to a certain degree, but you still get a big chunk of that distant hugeness that we all seem to love so much. Play it loud.
I'd also like to mention that another interesting trick I've found is to play the part, re-tune the Mellotron up or down from a half step to the full major third, and play the keyboard part again, transposing the key pattern on the keyboard to play in pitch with the original material. You now have 6,9,12, or 15-violins, because you are taking NEW "samples" and adding them together, each one having totally different ambience characteristics. If you are feeling ambitious, play different lines with each tuning and ambience, and do multiple passes to add in some of the small timing inconsistencies that a big group of violinists will have. The sound can get just entirely too huge, but if you want Mellotron that will part your hair, sandblast your car, and scare children and animals, try it.
That last trick is even pretty effective for huge Mellotron sounds recorded straight to tape with different artificial reverbs on each pass, but nothing comes out sounding quite like the loud Mellotron recorded from a distance in the big room with the artificial reverb. Smells like Wakeman in here...
www.tronsounds.com/songs/hvn.mp3