Can Proving Grounds decks be used in a Modified Champion Deck format event if regular Champion Decks sell out?

Ruling: While there is nothing inherently wrong with allowing Proving Grounds decks, there are several practical concerns that make them not ideal for Modified Champion Deck events.

Nuances:

• Proving Grounds decks sell for less than preconstructed Champion Decks, which creates a pricing disparity
• Proving Grounds decks only include one battlefield each (compared to Champion Decks)
• The starter decks are balanced against each other, so mixing Proving Grounds decks with Champion Decks may create balance issues
• Champion Decks come with a booster inside, creating prize equity issues if some players use Proving Grounds decks
• Players who practiced with Champion Decks may feel disadvantaged competing against different deck types