I would hook the 1/4 trs (balanced) to the back inputs 1-8 and switch the input to +4

you can plug it into the front input but you will have less headroom,  You will have to watch you don't clip the input. your gain settings will probably be low on the LA-610.Depending what kind of mic or what the source is. Is there a 1/4 inch output? if so use that.  It actually might sound better to use the XLR input of the UFX. You have to experiment.

The output of LA-610 (at least the MkII) is a balanced output at a nominal level of +4dBu, therefore with a nominal peak level of +14dBu (in American practice), but is capable of a maximum of +23dBu.   As the XLR on the front is only specified for a maximum of +12dBu, then it is quite capable of being clipped by normal operation of the LA-610.

The front TS input is unbalanced and high impedance (800Kohm) but will accept a specified maximum level of +21dBu.  Transformers like to see a lower value of impedance - somewhere between 1000 and 5000 ohms.  Also, a desirable 'sound' often required devices like th LA-610 to be driven heavily.

A good approach would be to place a 12 to 15dB attenuator between the output of the LA-5610 and the XLR input of the UFX. As recommended elsewhere in this forum, the Shure inline attenuators:

http://www.bhphotovideo.com/c/product/6 … uator.html

are convenient if you are unable to design and construct your own (and there are similar cheaper products available).   These also have the advantage of an input impedance of about 1500 ohm which could make the LA-610 feel fairly comfortable, and allow it to be safely driven to its full capabilities if required without exceeding the input capabilities of the UFX XLR in.

On a side note, if you are using it on the rear TRS inputs, the sensitivity should be set to Low Gain (maximum input +19dBu) rather than +4 (maximum input +13dBu) for the reasons outlined above.

Passive attenuators will not degrade the sound, unless they have faulty components or are badly built.   Metal film resistors and proper soldering practices will deliver perfect results.

That is correct assuming the pad's I/O impedance is conducive to the connected gear (might not want to use a Low-Z Mic pad on a preamp output as it will likely load the output down too low).  If in doubt - use your ears - good is good...

I regularly use passive DIY XLR-Barrel pads on the outputs of my colorful preamps to allow me to drive them into light (or heavy) saturation w/o clipping the AD inputs.  I have a variety of -6dB 1K pads, -6dB 600 Ohm pads, and a bunch of -14dB low-z Mic pads (they sometimes surprise me on the preamp transformer output - so make sure you experiment).  There aren't many A/D's that will take +28dBu and beyond w/o clipping - so padding is almost a necessary evil if you want to get these hot/colorful preamps in their "comfort zone"

I'll probably make some -9 or -10dB pads in the 1K Ohm (input) range as the -6dB's can be too gentle (not enough pad) and the -14dB Mic Pads are often too much and a bit too low on the impedance (like 220Ohms!) to keep the output transformers happy (loading them down kills some of the extreme high-end response).

Here is a link to a calculator to DIY custom pads:

http://www.grahambrock.com/t_h_pad.htm

According to the the LA-610 specs, and typical of the era, 600 ohms was the standard line transfer impedance.  Its actual source impedance is less than this, but the output spec says min. 600 ohm load.   Most of the time equipment like this was terminated in one or more line-in bridging inputs (nominal impedance 10K ohm).  1500 ohm load is a good compromise between the two cases, and while presenting a suitably low impedance to the XLR input.  But, as advised, listen.  It may perform more to your liking if it is actually terminated in 600ohm.  To achieve this, simply parallel a 1000 ohm resistor across the output of the LA-610, in parallel with the 1500 ohm attenuator.  The frequency and phase response of the output transformer is what is most likely affected by the termination impedance - but there, you can load according to taste!

+12dBu is the highest level that the XLRs will accept for 0dBfs, ie., without distortion.  Up to that level you will not exceed the headroom of the signal path.  It just so happens that with the preamp set to 0dB gain, that is the maximum level that the A-D converter can handle.

15dB pads are a very convenient value in this case, as this means that the XLRs inputs can then accept +27dBu, which means most (American) professional gear designed for a maximum output of +24dBu can be accommodated with 3dB to spare.  That is, the equipment output will go into distortion before the input.

Note that while most pads are designed for 'mic' levels, their ratings will accommodate 'line' levels.   Realise that +27dBu corresponds to 0.5 watt into 600 ohms.  Check the power rating specs for the attenuator.  But as an example, if the input impedance (resistance) of the pad is 1500 ohms, then a signal level of +27dBu (15V rms) across 1500 ohm is 0.15W dissipation.  As the pad is balanced, most of this will be dissipated across two series input resistors, so the power dissipated in each is about 75mW.  Most resistors have a minimum rating of a quarter watt namely 250mW, so in this sort of use they will be expected to handle line levels quite happily.  But check the specs, anyway.

And make sure phantom power is turned off - most analog devices don't like encountering 48V phantom on their outputs.

Trimming the analog outputs on the desk will slightly degrade the signal to noise ration of the desk's output, but if the reference output level can be set to 'consumer' levels (nominal level of -10dBV) this will work.  Remember that if accidentally driven hard, the input can still clip so always check the level in TotalMix or DigiCheck.

I can see that the metering is dual function now!