When I measured a 600 ohm audio source output connected to the 3458A through a 600 ohm feed-through at frequencies 5 Hz to 1.0 MHz, the DVM reading started to rolloff at 130kHz and at 1.0MHz the DVM display was >-6%. When I connected the Fluke 5700A wideband output to the DVM, the deviation over the 5 Hz to 1.0 MHz was <1.0%. When I connected the terminated 600 ohm audio source to a HP 400F, the analog meter deviated <1% over the 5 Hz to 1.0 MHz. What's happening with the 3458A? Is this a software issue?