There's more than one way to play the acoustic guitar, but you'd have a hard time convincing most folksies of that, so intent are they on preserving tradition and duplicating the style of this or that dead master. All the more reason, then, for a virtuoso folk instrumentalist as eclectic as John Martyn or as visionary as John Fahey.

Martyn, a Scotsman who's been playing guitar professionally for over a dozen years, started his career as a "Celtic folkie," one of hundreds of young minstrels specializing in revivals of traditional songs by the British Isles. But as shown by "So Far So Good" (Island ILPS 9484), a newly released "best of" collection, his interests have grown to include such American styles as jazz, blues and country - all of which are used to enrich his basic Scotch broth.

That means Martyn, who will make his Washington debut tonight at The Cellar Door, often finds inspiration in sources that other folkies neglect. He has said, for example, that "Head and Heart" takes as its starting point a triplet figure in a piece by jazz guitarist John McLaughlin and that coming across the echoplex caused him to revamp completely the way he had been performing the Skip James blues "I'd Rather Be the Devil."

It is a highly personal style of Martyn has developed - though even a purist would find it hard to quibble with his arrangement of "Spencer the Rover," a traditional English ballad. Working with musicians who share his willingness to experiment, such as former Pentangle bassit Danny Thompson, he has, with compositions like the shimmering "Solid Air" and the tone poem "Glistening Glyndebourne," created a music that goes beyond category.

Martyn's vocal style likewise defies classification. He's relatively amiable and straightforward on "May You Never," a simple, cheery little pop number that's been recorded by several other artists, but on "One Day Without You" and especially "Solid Air," his delivery is tortured and slurred in the manner of Billie Holiday. It's as close to scat singing as one can get while still singing words.

With both these vocals and his equally impressionistic musical accompaniment, Martyn creates an intense, emotionally charged ambiance. The moods conveyed vary from piece to piece, however: A dark bass line and stark guitar chords give a gloomy color to "Bless the Weather," while the interplay of voice with fiddle, autoharp and mandolin seems responsible for the lighter, more hopeful tone of "Over the Hill."

This kind of illusionism has been practiced by John Fahey, the American University philosophy and religion graduate who will follow Martyn into The Cellar Door this weekend, for an even longer time. What's more, Fahey's sleight-of-hand is entirely of the solo variety - there are no voices or accompanying musicians whatsoever on the new retrospective album called "The Best of John Fahey: 1959-1977" (Takoma C-1058).

Fahey's music stresses simplicity, but nonetheless seems remarkably rich and evocative, especially on original pieces such as "When the Spring Time Comes Again" and "The Sunny Side of the Ocean." Though he calls his style "American primitive guitar" Fahey has given his music depth, warmth and striking emotional resonance by means of frequent transpositions into minor keys, unorthodox tunings and multiple melody lines picked simultaneously.

"Like Segovia, who used the guitar techniques of Spain to make arrangements of classical compositions by composers like Bach," Fahey once said, "I use the techniques of the United States and a few I think I invented myself to play my own songs." Or, to be exact, mostly his own songs - Fahey also applies his philosophy, with just as much success, to the hymn "In Christ There's No East or West" and blues such as Bukka White's "Poor Boy" and W. C. Handy's "St. Louis Blues."

That's only fitting, because the original inspiration for Fahey's music was the blues - whose influence can be detected even in such fancifully titled pieces as the bottleneck instrumental "Dance of the Inhabitants of the Palace of King Philip XIV of Spain." What has made Fahey a unique stylist - and prompted Guitar Player magazine to publish transcriptions of and commentary on the pieces here - is the way he's built on this foundation, warping it to conform to his own peculiar outlook.