I don't know if it was NRG, but it was most likely a studio because the songs are very high in quality, and at that point and time in his life, I personally think that Mike wouldn't have been able to afford the equipment required
someone should add this to the F.A.Q once its answered i have no idea, but i think xero was in mikes bedroom, and u can see the quality difference between xero and HTEP
But still the EP isn't the same quality than HT. They maybe recorded in some unknown studio, there are a lot of places where you can record and have a pretty good sound.
From the Hybrid Theory EP booklet: "This was a CD that we recorded in a local studio and at home at Mike's apartment in 1999. We remembered sitting around at Mike's late at night, playing, singing, screaming, and annoying the neighbors for many months to record this seven-song demo. Upon its completion, only about 1000 copies were made."
Thanks Will . So my answer made some sense. Mike maybe had new equipment, because the sound on the Xero tape was really... really... no comment .