I finally have a Borsao ally!!! Thanks Megia!
You mention a lot of interest wines and regions. Jumilla, where the Olivares is from, has some bodegas making very good sweet wine from Monastrell grape. Traditionally many Spanish wines were sweet, and it's good to see them making a comeback.
Muga and Gurpegui Muga are two entirely different wineries. There may be some family relationship there since both have been around for a long time, but I haven't found any. Wines from Gurpegui Muga are definitely more economical (and are perfectly sound), but Mugas are rated as some of the best Riojas around.
As for D.O. Rueda, well you are partial to their white wine because that is still their best bet. As for the Marqués de Riscal, I'm glad to hear you like it. Actually Marqués de Riscal was the pioneer in that region. In the 1970s, ruedas were very different wines (almost sherry-like), but it was Marqués de Riscal, in search of a new outlet for its white wine market, which moved into town and changed the direction of that region forever. So in a sense, we have Marqués de Riscal to thank.
As for rueda reds? There's not much available, but you might want to ask around for one of these two: Vacceos Crianza(tempranillo) or Monasterio de Palazuelos Crianza (thought this was pretty good). They are genrally good values.