I've been reading the recent Allentoft paper. Figure 2b shows admixture data for ancient DNA samples in Europe and the Pontic-Caspian steppe.
Population genomics of Bronze Age Eurasia
Allentoft et al
Volume: 522, Pages: 167–172
As it turns out, at least one of the Beaker samples from this paper has the y-dna haplogroup R1b-U152/S28. That is particularly interesting, because, according to a reference mentioned on wiki for Bell Beaker "A review of radiocarbon dates for Bell Beaker across Europe found that some of the earliest were found in Portugal, where the range from Zambujal and Cerro de la Virgen (Spain) ran between 2900 BC and 2500 BC".
The immediate R1b haplogroups upstream from R1b-U152 cluster in Iberia, which fits nicely with what we know about early BB.
Another thing I would note is that with the ydna haplogroup R1b-U152 now known to be in Bronze Age BB ancient DNA, and with the ydna r1b Z2103 (Balkans) /L51 (Alps/Western Europe) split upstream from this, it fits nicely with the R1b haplogroup expanding into Western Europe well before the Bronze Age.
Looking at the above Figure 2b, it is evident that Yamnaya has some admixture from North Eastern Europe (the light blue component) that does not appear in the Beaker Samples. So we can pretty much rule out these Bronze Age Yamnaya samples in the Allentoft paper as being the direct ancestors of the Beaker Samples. That is also the case for the Corded Ware samples. They can't be the direct ancestors of Bell Beaker either, and for the same reason.
Looking at the Bronze Age Montenegro sample, it doesn't have the light blue ancestry component. It also looks very much like the Bell Beaker samples, except for having slightly more of what Allentoft et al. call the Caucasian component.
A significant upland Balkan contribution to Bell Beaker would fit with the timing of the Z2103 (Balkans) /L51 (Alps/Western Europe) split and with the idea that during the Neolithic, BB ancestors from the uplands of the Balkans followed a mountain route along the foothills of the Dinaric Alps/Alps/Pyrenees into Iberia and Western Europe.
If I were working in ancient DNA studies, I'd definitely be out there trying to find some upland Balkans ancient DNA from the Bronze Age and Neolithic.