You can find many super small/cheap 600 ohm signal transformers on ebay, or if you want to spend more, your local music or Guitar Center store should have XLR to TRS adapters with the transformer built in, as does Amazon.
This may be worth playing with if you don't like wiring up your own using transformers from ebay.
Yes Art, Like Neal said if you need to go from a device that has a "Balanced" output like an external mic preamplifier to the UN-Balanced input of the 6400/M you will want to use an audio isolation transformer. You can use the one Bill recommends because it gets the job done, is low cost and is more than sufficient for ham radio. ( https://www.amazon.com/PYLE-PRO-PHE400-Eliminator-2-Channel-Outputs/dp/B00BARTW42/ )
Keep in mind some of the of the lower end boxes like these tend to use transformers that are not audiophile grade and can have a bit of low roll off at @60 down and also some attenuation in the top end at around 16khz and up. This should not matter for normal SSB but if you were more into ESSB you might want to go for the better transformers.
You could optimize even more by using higher quality transformers like Lundah or Jensen and try to match input and output impedance with equipment as much as possible for best audio fidelity.