probably mine to address, as it involves fixed volume which I've been working on recently.

though there hasn't been enough information posted about the user's setup, it sounds to me that the user is using SBS 7.6? Please correct if this is in error, as this is important info.

Checked the logs from the user.
serverstatus-message returns version 7.5.1

This is the setup of the particular user copied from his mail:
"Squeezecenter running on ReadyNas NV+
4 Squeezeboxes
- one SLiMP3 using a wireless bridge
- one Squeezebox v1 using built in wireless
- one Squeezebox Classic using built in wireless
- one Squeezebox Classic using powerline networking

When I use SqueezePad, I can see the cover art and "Now Playing" for all three top listed players. When using SqueezePad to control the 4th listed SqueezeBox, I only get the "generic" screen, without cover art or "Now Playing" info."

Player 4 was the one with fixed output level. My 7.5.1 server running on a SheevaPlug returns a proper '0' at this place when I use the fixed output level.

from 7.5.1 serverstatus response code:

        if ($client->hasVolumeControl()) {
                # undefined for remote streams
                my $vol = $prefs->client($client)->get('volume');
                $vol += 0;
                $request->addResult("mixer volume", $vol);
        }

this code block is also unchanged in 7.6 SBS

I'm puzzled by this bug, because the $vol += 0 is an explicit step to force the data type to be a number. We do this specifically for Lua on the other side, that is very particular about data types (unlike Perl). 

What you're seeing in that output, 'nan', is an error indicating bad data type (nan = "not a number").

But NaN (like +Infinity and -Infinity) is still a number type (at least in JavaScript, maybe Perl too), so maybe the +0 doesn't help here ?

I used this site to help me 'create' a NaN in Perl to fake a JSON with a NaN.
This at least isn't handled in the (Perl) JSON Converter, so 'gets through'.

http://stackoverflow.com/questions/1185822/how-do-i-create-or-test-for-nan-or-infinity-in-perl

I didn't check though what influce a '+0' could have on a NaN, but I guess this doesn't help.

I also read that Perl support for NaN might be platform specific, so it could be a special case for ReadyNas.

Just did a small test with a snippet faking a NaN:

-----
        if ($client->hasVolumeControl()) {
                # undefined for remote streams
                my $vol = $prefs->client($client)->get('volume');
                $vol=-sin(9**9**9);
                $vol += 0;
                $request->addResult("mixer volume", $vol);
        }
-----

I still get the following JSON on the client so the 'trick'/hack with the $vol+=0 doesn't work for NaN that might be introduced earlier on for some reason.

------
{
    channel = "/15bdeb95/slim/playerstatus/00:04:20:17:1d:38";
    data =     {
        "mixer volume" = NaN;
        mode = stop;
        "player_connected" = 1;
        "player_ip" = "192.168.1.10:45967";
        "player_name" = Wohnzimmer;
        "playlist mode" = off;
        "playlist repeat" = 0;
        "playlist shuffle" = 0;
        "playlist_tracks" = 0;
        power = 1;
        "seq_no" = 0;
        signalstrength = 41;
    };
    ext =     {
        priority = "";
    };
    id = 8;
}
------

Andy, now that it's becoming clearer where the break in this JSON, I'm thinking it's not really my area. Reassign as you see fit, but I think this is yours.