If you only need to operate up to 1GHz, one cheaper option is to make an external wideband preamp (eg 30dB gain) for the HP 8594E and then offset the amplitude reference level by the gain of the preamp. It should be possible to make a preamp with a sub 4dB noise figure and flat gain up to 1GHz using cheap MMIC parts from Analog devices or Minicircuits.
The input VSWR up to 1GHz will probably be better than the 1DS preamp as well.
It does look like your HP 8594E is a bit noisier than expected though. -90dBm with a 1kHz RBW is quite poor. Is this with 0dB input attenuation?
Note that if you want to measure noise more accurately, it's best to use the 1Hz noise marker function in the 8594E. This will be better than using a normal marker and a 1kHz RBW.
