Pentax XW 5 mm has 20mm of eye relief with a AFOV of 70 degrees. The Baader Morpheus 6.5 is stated at 18.5mm with an AFOV or 76 degrees.
On those nights you can use the Pentax 5mm. You'll be glad you have it. You don't want to planetary observe with a barlow. Adding lenses and air to glass surfaces cause light scatter. Light scatter causes loss in contrast, and loss of contrast causes loss of detail.
I created a chart that calculates the functionality of my eyepieces. Every eyepiece will always be useful somewhere. One of my scopes is a 12' dob like yours but you don't always need to be looking through the 12' to appreciate a Pentax eyepiece.
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