Over my career, I've used a lot of HP 346A noise sources (5.5dB ENR) but only a few examples of the 346B (15dB ENR).
?
In my experience, the input VSWR of the 346A (in both hot and cold states) is usually very low across 10MHz to a couple of GHz. A typical VSWR figure might be 1.02:1 across this frequency range. A good example might achieve a VSWR of < 1.01:1 at 10MHz. At frequencies below 10MHz, the VSWR will gradually degrade, but I don't think it will get much worse than about 1.06:1 even below 1MHz.?
?
Probably the biggest issue with the 346A will be finding an instrument that formally supports it below 10MHz. In terms of noise figure measuring performance, the performance of a typical analyser will degrade quite a bit below 10MHz in various ways. This makes it difficult to support a noise source with a low (5.5dB) ENR like the 346A. You may be better off using a 346B with (say) a 6dB attenuator fitted to it to lower the 15dB ENR a bit and also improve the source match. This might achieve a better compromise for use below 10MHz.
?
It's a while since I used a 346B so I've forgotten the typical input VSWR vs frequency. The VSWR will be higher for the 346B because it has less internal attenuation after the noise diode. One option is to fit a low Q matching network at the input of the 346B as well as an external attenuator. This would hopefully deliver a decent ENR of maybe 9dB as well as providing a good source match below 10MHz. This should improve the overall uncertainty when using the noise source although you would have to find a way to determine the ENR of such a setup when used below 10MHz.
