Left and right speakers have different surroundings... Right speaker is close to a passage and most of the area behind is open... Left speaker, on the other hand has a metal plate covering the radiator and a glass window behind. Totally different... That's where I feel the GLM calibration helps a great deal to reduce those differences.
Regarding the Hofa basstrap baby. It is not really small... I got it in cream (the lightest option they had) to minimize its visual presence. Sonic effect where positive overall, not night and day (I think I need more room treatment), but it tamed some resonances that tended to smear the soundstage and make the left side a little more bass heavy... Subjectively, the sonic presentation was more textured with greater mid range definition. Precision audio at home is almost impossible unless you are willing to sacrifice livability... I've attached the calibration curves without the baby trap (GLM 4.0 Beta).
Here are some pictures to illustrate this post:
Hofa Basstrap Baby
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Right 8351B
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Left 8351B
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The GLM calibration without the Hofa basstrap baby. I had a real Christmas tree in that corner:
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And with the Hofa basstrap baby (also shared in a previous post):
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