Does this work for connecting the sound output from a PC or MP3 player into a PA system using a Stereo 1/8" TRS to Mono 1/4" TS cable?

Yes that should be fine! :) You would also need an XLR to connect the DI Box to the PA.

I want to record digital piano and play live at church. Should I get Pyle-Pro PDC21 for $8 or is this significantly better?

A normal DI box is $50 so I wouldn't go too much lower than this. These seem to work well. I've only had one go bad in the past year or so and I purchased like 8 for my church.

Can this reduce a speaker level signal to a line level signal? E.g. if I wanted to take a speaker level and connect it to a mixer.

YES. The other answers are not accurate. This device as well as many other DI Boxes can make a Line Level 'copy' of the high power signal going to a speaker. This is a common use however this yes comes with a major caveat. The source (1/4 " INPUT) can be a highly amplified signal, such as the speaker output of a 100W guitar amplifier. The output signal (XLR OUTPUT) will be what the mixer wants (Line Level). CAVEAT: There must be a load connected to the THRU (Parallel OUT on this device). This load can be a speaker or speaker load simulator, basically what you would normally connect to the amp. Without a load connected, no signal flows and nothing will be output. Worse yet, without a load connected, many amps will fry. Amp to DI box to Speaker(1/4" OUT) and Mixer (XLR OUTPUT) is commonly used by guitarists. This allows the PA Mixer or Recording Audio Interface to get all the good sound that the guitar signal chain has built, only missing coloring from the speaker cabinet. If needing your speakers to be silent while still getting a good output on the XLR, adding a decent amp Attenuator or Load Simulator after the DI Box can reduce the power reaching the speaker to or near zero. Note: Many higher end Attenuators and Load Simulators have a built in DI Box.

can i use this to hook up my 6505+ to my m-box?

If you have the combo version, it has an XLR out already, so no DI required. The head version doesn't seem to have this option, so you could use the preamp out, but I'm not sure if you could hear a difference between using a DI or running the 1/4" into the line level input of the M-Box. You'll lose the tone you get from the power amp tubes either way. The main point of a DI is to convert a 1/4" instrument signal to a balanced XLR signal if you're going into a stage snake. Effectively running a 50' or 100' 1/4" cable will generally introduce noise and reduce signal quality, so the DI is a great option. I've used this passive DI box in live situations for the past year, and it's worked like a champ. I run my tracks (from an iPod) through it.

ASIN	B000KUA8G6
Amazon Bestseller	#2,624 in Musical Instruments ( See Top 100 in Musical Instruments ) #1 in Stage DI Units & Patch Bays
Color	Black
Color Name	Black
Color Screen	No
Compatible Devices	Smartphone
Control Method	Voice
Customer Reviews	4.6 4.6 out of 5 stars (2,068)
Date First Available	December 2, 2007
Included Components	Di400p Compact Direct Box with Parallel
Is Discontinued By Manufacturer	No
Is Waterproof	False
Item Weight	230 g
Item model number	DI400P
Material	Metal, Plastic
Material Type	Metal, Plastic
Model Name	DI400P
Mounting Type	Tabletop Mount
Number of Items	1
Power Source	Wire
Product Dimensions	3.5D x 12W x 6.6H cm
Recommended Uses For Product	For Smartphones and Tablets Laptops
Speaker Size	4 Inches
Speaker Type	Outdoor
Subwoofer Diameter	4 Inches
Surround Sound Channel Configuration	2.0
Unit Count	1 Count
Wireless Communication Technology	なし。

Behringer DI400P Direct Passive Type Compact Size

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