If you need a very low band source below 10 MHz, you could conceivably use one of these heads with a high impedance buffer amplifier. That would take care of the LF roll-off from the coupling cap. You'd need to account for the gain scaling and additional noise of the amplifier of course, but it should be straightforward. Down at the low frequencies you won't be worried about impedance matching at the input, just straight voltage gain (like unity), and let the output take care of matching to 50 ohms or whatever.
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A simple emitter follower or JFET follower or totem-pole (or even an opamp) should easily perform the Z transformation over reasonable bandwidth. Depending on circuit design, you'll still probably have an output coupling cap, but you can make it as big as you want. With opamp circuits, you can go with DC output coupling to get way down, leaving only the input side to limit the LF response.
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With these 34X noise sources, I think it's only one stage, with a noise (avalanche) diode and coupling network, so probably only one dominant capacitor for the low end.
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Ed
