The pieces in Shiurim l'Zecher Aba Mori are interesting because, while the shiurim are oriented towards halacha/lomdus, the Rav develops and dwells on certain philosophical ideas that emerge directly from the halachic discussion at hand. Examples: ideas on the meaning of Shabbos emerge from the shiur on kiddush, ideas about teshuvah can be found in the shiur on mechikas Hashem, etc. This element is completely absent from the writings of R' Chaim or the Brisker Rav. I am not sure if the Rav paid more attention to philosophical themes because of the venue (a public lecture is different than a shiur given to close talmidim ) or whether it was more generally part of his thinking process.
These philosophical digressions and shadings are interesting because the Rav follows (obviously) the Brisker tradition of generally eschewing both mysticism and mussar/machshava. The universe of Brisk is defined entirely by the boundaries of the "four cubits of halacha." Perhaps Halakhic Man and Halakhic Mind should be read as the Rav's attempt to integrate his own philosophical quest and interests into these self-imposed boundaries. One passage from Shiurim l'Zecher Aba Mori (vol 2 page 173, excuse my rough translation) struck me as especially illustrative of this narrow Brisker focus:
I have always been troubled by the role and position of the prophet. On the one hand, we rule that a navi is prohibited from introducing innovation in halacha, from adding or detracting "even the crown of a letter yud;" on the other hand, Hashem communicated with the nevi'im, they prophesied, and their prophecy was written for all future generations. What purpose did their prophecy serve, given that they could introduce no halachic chiddush? True, they rebuked the nation, and to give rebuke is certainly one of the reasons prophets were sent. But still, I am troubled by the notion that their message should be completely devoid of halachic content.
The Rav simply could not fathom that there could be a "cheftza" of the dvar Hashem seperate from the narrow universe of halacha. I read this to my son and he was dumbstruck. Kushya m'ikara leisa if you have not bought into the world of Brisk. My wife reminded me that the gemara itself is suggestive of a similar idea -- Chazal tell us that had the Jewish people not sinned, they would only have needed the five books of chumash and sefer Yehoshua (to know the boundaries of Eretz Yisrael). In an ideal world all that exists is halacha.
The Rav goes on in that shiur to answer his question by saying that indeed there is halachic content to the words of navi. A person has an obligation to imitate G-d. The nevi'im teach us G-d's midos so we can fulfill our halachic obligation of modeling our behavior accordingly. Rather than acknowledge a universe outside the narrow world of halacha, the Rav reduces that outside universe to fit into the four amos of halacha. Just as the Chazon Ish in Emunah u'Bitachon erased the separation between mussar and halacha by teaching that ethics is compltely defined by the world of isur v'heter (i.e. there can be no such thing as an ethical wrong which is permitted, or an ethical good which is prohibited -- halacha is the arbiter of just/good), the Rav here reduces all mussar and ethics to kiyumim that fit into his narrow halachic universe.
Some will definitely find the focus of Brisk is too narrow, and their souls will yearn for some poetry to go with the prose. For others, the beauty of the rigor and analysis of Brisker torah forms its own form of poetry.