Solution to 11A has been deliberately left unsolved and is to be answered only by a non-regular / novice commenter, with proper annotation. Those who have answered earlier in the week, please give others a chance.
ACROSS
8 Sit oddly before old sound system (6) STEREO {SiT}{ERE}{O}
9 Old viral poem about lost love (8) PRIMEVAL {VIRAL+PoEM}*
10 Better attendant? (8) CROUPIER [CD]
11 Tie note encircling employee's original resume (6) R?O?E? (Addendum - REOPEN {R{Em...r}OPE}{N}
20 Son went to get a little ornate instrument used in vehicles (6) SPEEDO {S}{PEED}{Or...e}
22 Are terms amended to produce new version of old recording? (8) REMASTER*
25 Stylish mark covering main part of the cloth (6) INSEAM {IN}{SEA}{M}
26 Poem by retired salesman is ill-natured (8) PERVERSE {VERSE}<=>{REP<=}
27 Dismiss beautiful woman without sex appeal and talent, primarily (8) BELITTLE {BEL{IT}{Ta...t}LE}
DOWN
3 Dutch man at home wearing spectacles and a loose cloak with wide sleeves (6) DOMINO {D}{O{M}{IN}O}
4 Condemn attack by Britain after leaders of Ukraine pressurised (7) UPBRAID {B}{RAID}<=>{Uk...e}{Pr...d}
5 Title Rob used for book containing words of Opera? (8) LIBRETTO*
6 Monument spread over Bank of Missouri (8) MEMORIAL {ME{M or I}AL}
7 Request Bill to interrupt German person who is dishonest (3,3) BAD EGG {B{AD}EG}{G}
15 Wild democratic movement to acquire dominance at the centre (8) FRENETIC {FRE{d..iNa..e}E}{TIC}
16 Crime novel by Christie accepted by English town, needing no introduction (8) ENORMITY {E}{N or M}{cITY}
18 Dislike American style (8) AVERSION {A}{VERSION}
19 Outing permitted for a group of three (7) TRIPLET {TRIP}{LET}
21 Hand tool price changed around beginning of November (6) PINCER {PI{No...r}CER*}
23 Injure newcomer endlessly and put to death (6) MARTYR {MAR}{TYRo}
24 Sad to invite that man without originally offering a drink (6) SHERRY {So{HE}RRY}
Reference List
Before = ERE, Old = O, Note = N, Lawrence = TE, Went = PEED, Son = S, Stylish = IN, Mark = M, Sex appeal = IT, Dutch = D, Man = M, Spectacles = OO, Britain = B, Bill = AD, German = G, English = E, American = A