Could you tell me about this

[TamaSabian]

I´ve noticed that my mixes are almost within 150-250 hz. I talked about this with a friend of mine who has a studio and he told me that it could be my room (not sound treated) or my "power amp", well it isn´t a power amp, it´s an 79' AKAI 803A stereo receiver (not so shure about the model) that I use with a pair of Proto J.
What can I do to detect where´s the problem???.
Please tell me about this and if my friend is right.

Thanks in advance

TS