I read in p. 2388 of Handbücher zur Sprach- und Kommunikations-wissenschaft (De Gruyter Mouton) that warm stellen and warmstellen are pronounced the same and are prosodically indistinguishable in:
die Suppe warm stellen (Q1)
die Suppe warmstellen (Q2)
What Duden says is in accordance: (bold a shows the stress location):
warm stellen
warmstellen
However, when I looked for the place of stress in the verb stellen I found that it is located in first syllable as is expected: stellen (As Duden says).
The question is why should stellen loose its stress in the sentence marked as Q1 above? It is the verb in that sentence and warm is the adjective. They are not bound into a single lexeme; then why should stellen have lost its stress? In the case of Q2, I can say that we have a single lexeme, warmstellen. Then, it's not surprising that we have only one stress peak which is placed on the first syllable.
Thanks,