help me find the root of this noise problem

[preachers]

i have just experienced a noise problem with my mics or micpres.

i have 2 mics and 2 micpres
they're:

mic1:u87
mic2:AT4050/cm5
pre1: tubetech mp-1a (the tube one)
pre2: AMEK 9098 (the transistor one)

fisrt time, i connect a u87 to my tubetech mp-1a, it was all fine, but i found u87 was not suit the singer i recorded, then i change the mic to at4050, and a big hum appeared when the signal level is ok. i think it might be the mic problem, this time i change the pre, but it with AMEK 9098 was fine, very quiet, then i change the mic to u87 and it was all fine with 9098 too.

the picture below is the big floor noise spectrum when use 4050 with mp-1a

now, i'm wonder where is the root of this problem? mic or pre????

any suggestion would be appreciate

[preachers]

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