Can't say I know Doom Metal well enough as a genre to comment - the 16th note tremolo drone is kinda thrashy in flavour, but the lyrics of Erlkoenig are Prog Rock, surely? There is a little-known Krautrock band from the early 1970s called Erlkoenig - and their
one and only album is actually rather good.
Tod und das Machen is largely in D minor - for centuries, traditionally the key of Death (and used by Mozart for both his haunting Requiem and his "horror story" Don Giovanni), so I guess it kinda qualifies...
After all, it was metal uber-God Nigel Tufnell who said "I find D minor to be the saddest of keys..." as he demonstrated his incomplete trilogy "Lick My Love Pump" to Marty DiBergi.
Seriously - it's always good to dig out the Classics - can you provide a stronger link? Is there a particular Doom band whose music directly links back to Schubert, or is this a more generic thing?