Strange, i’ve yet to experience any popping, underruns on my Shield 2.
Some things that might be worth checking:
- If USB powered, try another PSU. May be causing interference on Shield 2
- Try removing the UPS, again, may be causing feedback on the Shield 2
Failing that, lets compare your mixer settings, here are mine:
root@DietPi:~# amixer -c 1
Simple mixer control 'DSP Program',0
Capabilities: enum
Items: 'FIR interpolation with de-emphasis' 'Low latency IIR with de-emphasis' 'High attenuation with de-emphasis' 'Fixed process flow' 'Ringing-less low latency FIR'
Item0: 'Low latency IIR with de-emphasis'
Simple mixer control 'Analogue',0
Capabilities: pvolume
Playback channels: Front Left - Front Right
Limits: Playback 0 - 1
Mono:
Front Left: Playback 0 [0%] [-6.00dB]
Front Right: Playback 0 [0%] [-6.00dB]
Simple mixer control 'Analogue Playback Boost',0
Capabilities: volume
Playback channels: Front Left - Front Right
Capture channels: Front Left - Front Right
Limits: 0 - 1
Front Left: 0 [0%] [0.00dB]
Front Right: 0 [0%] [0.00dB]
Simple mixer control 'Auto Mute',0
Capabilities: pswitch
Playback channels: Front Left - Front Right
Mono:
Front Left: Playback [on]
Front Right: Playback [on]
Simple mixer control 'Auto Mute Mono',0
Capabilities: pswitch pswitch-joined
Playback channels: Mono
Mono: Playback [on]
Simple mixer control 'Auto Mute Time Left',0
Capabilities: enum
Items: '21ms' '106ms' '213ms' '533ms' '1.07s' '2.13s' '5.33s' '10.66s'
Item0: '21ms'
Simple mixer control 'Auto Mute Time Right',0
Capabilities: enum
Items: '21ms' '106ms' '213ms' '533ms' '1.07s' '2.13s' '5.33s' '10.66s'
Item0: '21ms'
Simple mixer control 'Clock Missing Period',0
Capabilities: enum
Items: '1s' '2s' '3s' '4s' '5s' '6s' '7s' '8s'
Item0: '1s'
Simple mixer control 'Deemphasis',0
Capabilities: pswitch pswitch-joined
Playback channels: Mono
Mono: P▒▒▒{&▒> [on]
Simple mixer control 'Digital',0
Capabilities: pvolume pswitch
Playback channels: Front Left - Front Right
Limits: Playback 0 - 255
Mono:
Front Left: Playback 201 [79%] [-3.00dB] [on]
Front Right: Playback 201 [79%] [-3.00dB] [on]
Simple mixer control 'Volume Ramp Down Emergency Rate',0
Capabilities: enum
Items: '1 sample/update' '2 samples/update' '4 samples/update' 'Immediate'
Item0: '1 sample/update'
Simple mixer control 'Volume Ramp Down Emergency Step',0
Capabilities: enum
Items: '4dB/step' '2dB/step' '1dB/step' '0.5dB/step'
Item0: '4dB/step'
Simple mixer control 'Volume Ramp Down Rate',0
Capabilities: enum
Items: '1 sample/update' '2 samples/update' '4 samples/update' 'Immediate'
Item0: '1 sample/update'
Simple mixer control 'Volume Ramp Down Step',0
Capabilities: enum
Items: '4dB/step' '2dB/step' '1dB/step' '0.5dB/step'
Item0: '1dB/step'
Simple mixer control 'Volume Ramp Up Rate',0
Capabilities: enum
Items: '1 sample/update' '2 samples/update' '4 samples/update' 'Immediate'
Item0: '1 sample/update'
Simple mixer control 'Volume Ramp Up Step',0
Capabilities: enum
Items: '4dB/step' '2dB/step' '1dB/step' '0.5dB/step'
Item0: '1dB/step'P.S. Of course, one should not run dietpi-update ( or any dietpi-xxx for that matter) after upgrading to stretch: apt winds up in an unresolvable state (no way to upgrade or downgrade packages anymore) - at least to software developers like me, who are apprentice sysadmins
Our patch system assumes if your running Stretch, its our RPi image. This is the only Stretch image we currently offer.
So yes, avoid updating if your manually switching to Stretch, at least until we can offer an image.
Also, dietpi-backup is a great way to create a system restore point, just incase your brave enough to dietpi-update your Stretch installation 
, it offers a way to revert your system.