Concentrating on the Amen of the answers:

Some Poskim[1] rule that one who says a blessing is to concentrate on the Amen said by the people who heard the blessing.[2] Other Poskim[3] however rule that it is not necessary to do so.[4] Practically, Admur rules like the former opinion[5], and so concludes the Rebbe.[6]  

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[1] Admur 167/14; Rama 167/2; Or Zarua 1/102 and 178 based on Yerushalmi Brachos 7/3; P”M 167 A”A 36; Likkutei Sichos 35 p. 220

[2] The reason: As the Amen is also part of the blessing [Darkei Moshe 167/4; Admur 167/3; Gra 167; Rashi Brachos 47a] and through answering Amen the blessing of the person receives greater importance, and hence it is initially proper to intend to fulfill one’s obligation with the Amen said by the listeners. [Darkei Moshe ibid; Or Zarua ibid; Yerushalmi; M”B 167/20]

[3] See Biur Halacha 167/2 “Vehamevarech” that so is implied from Lechem Chamudos and Shiyurei Kneses Hagedola, and that aside for the Or Zarua, no other Posek mentioned such a law. This ruling of the Rama [regarding concentration to be Yotzei the Amen] is omitted from Admur in 167/3 and is possibly negated from his wording there, as explained next, however in Admur 167/14 he explicitly rules like Rama ibid. See Likkutei Sichos 35 p. 220 footnote 41

[4] The reason: As there is no obligation for one who says a blessing to say or hear Amen, and on the contrary one who says Amen to his blessing is considered a fool. [Biur Halacha ibid]

[5] Admur 167/14

Ruling of Admur in 167/3: Admur 167/3 completely omits this ruling of the Rama and Or Zarua regarding concentration. Furthermore, Admur stipulates the entire law of delaying cutting the bread until the Amen has been completed by majority of the congregation to only a case in which the listeners are fulfilling their obligation with this blessing of Hamotzi, and plan to eat bread on its basis. Now, although Admur records the wording of the Darkei Moshe and Or Zarua ibid that Amen is part of the blessing, nevertheless Admur learns that this only applies when the person saying Amen is fulfilling his obligation with the blessing. Perhaps the reason for this is because in truth Admur rules there is no need for the person saying the blessing to concentrate by the Amen, not even in a case that the person saying Amen is fulfilling his obligation. However, nevertheless, he may not cut the bread until the Amen is complete, being that the Amen of one who is being Yotzei a blessing is part of his blessing and the bread is to remain a Shaleim until one completes the blessing. [See Likkutei Sichos 35 p. 220 footnote 41 that leans to learn this way in Beis Yosef and Admur ibid based on their wording, although then brings the real Girsa in Orchos Chaim which implies like Rama and Or Zarua] However, despite the above Diyukim, in 167/14 Admur explicitly rules like Rama and Or Zarua ibid that one must concentrate on the Amen of the listener. Hence, one must conclude that Admur on the one hand holds that one must concentrate to the Amen, and on the other hand holds that one may cut the bread if no one is being Yotzei, as the main thing is that one does not eat the bread prior to the Amen. This would explain why Admur 167/3 changed the wording of the Orchos Chaim and wrote not to cut the bread rather than not to eat the bread. Accordingly, though, Admur argues on the reason of why he cannot break the bread prior to the completion of the Amen and it is not because he needs to be Yotzei it, as he can be Yotzei it even after the breaking prior to the eating, but rather because of the listeners. Vetzaruch Iyun!

[6] Likkutei Sichos 35 p. 220