When I'm testing with REW analysis suite, I use an interface that has 4x 'line input' and 'line output' balanced connections ...
One pair of I-O is for REW analysis .. ie. a main 'DUT' send and return, and also the 'Loop Back Reference' send and return.
The other pair of I-O I use to 'audition' stereo audio, to an amp and speakers .. could be a media player - most of the time [long, long hours at the bench

] - or a favourite intrument channel or so .. which I can 'software' patch into the 'measurement' chain when I want .. for those 'spot' measures.
It helps nicely to have 'a pair' for testing DUT and 'another pair' to listen on the speakers .. while you go .. but completely seperate from the 'signal generator' at all times

And, apart from that, I do like to use an SPDIF pair too, for digital perspectives and to drive my 'Finalizer'

BAsically a 4x4 bal analog with 2x2 digital .... if they have a 'mic input' and/or a 'DI' on front, all the better [for test and measure].
...
As far as the input and output range goes, plenty of cheap m-audio do balanced I-O up to around 10Vpp sine with really high fidelity as 'a loopback'. Which is fairly decent, I think.
Much, much higher fidelity than almost any reasonable cost hardware signal generator box out there.
For anything I need to drive higher than around 10Vpp, I use a step up 'test bench' transformer *and/or* a solid-state amp module ... I'm talking 100 dollars for a bunch of Edcor [good performance and value for matching transformers] and a few ebay diy 'op-amp' modules, in the budget [as well as].
-> ie. for various kinds of testing of intermediate level and distribution amps and so on especially for bal/unbal bs.
I also like the Behringer Headphone Amp 1ru box for this role, in conjunction with the afore-mentioned [said] Edcor{s} for similar overall money.
They [ebay op amp modules] give you 'balance-debalance' .. and 'straight wire gain with Alps fader [quiet and crackle free] ' level control.
These modules let me measure some of the finer details of things quickly [I have a limited attention span] and easily [low threshold for the ditchin']. Again, some Behringer gear fits this 'test bench' role nicely for similar dols.
...
For anything I need to receive higher than around 10Vpp, I use a diy 'psuedo balanced attenuator' box for the 'single ended' stuff and a basic attenuator for the balanced.
-> ie. mostly for tube biz of all kinds, and for the probing [with a suitable safe probe

] of course.
If the power is still higher, then I can link in a 'load box' into the above scenarios and a solid state power amp module with still very high levels of 'measurement channel integrity'
..
Covers most audio ranges with quite amazing fidelity and a good launching point for the more 'hi resolution' endevours.
My 100 dollar m-audio box Profire 610 does all the above, in a way that if you accidently 'bork it', it's not too big of a deal.

Admittedly, it is old so one needs to consider their hardware platform.
I think I get something like 0.0015% at least, in most testing scenarios for my best 'measurement channel' in real life situations.
[firewire in my case - up to fs 96KHz with no probs]
For more modern money, I'd love to try some of the nice ones out there.
They do do an order of magnitude better for 10x the investment - but mind you don't fry it with tubes!
